What Is “Normal” Infant Sleep? Why We Still Do Not Know

What we know about infant sleep

In the early 21st century, most Western experts agree on the following characteristics of infant sleep (Heraghty et al., 2008): (1) Infants have different electroencephalography (EEG) patterns than adults, including active sleep (analogous to rapid eye movement (REM) sleep in adults), quiet sleep (analogous to slow-wave sleep (SWS) in adults), and an unclassified type of sleep that is not analogous to any type of adult sleep. (2) Full-term infants spend about 50% of total sleep time in active sleep, and this percentage is reduced over time to about 25% by six months of age. (3) Infants spend about 17 hours per day asleep at birth, distributed throughout the day and nighttime hours; by one year infants’ sleep is reduced to about 14.5 hours, most occurring at night. (4) Circadian rhythm develops by about four months of age, based on a day–night cycle as well as environmental influences. All agree that infant sleep undergoes many changes throughout the first year.

Recognizing that Western infants are not the only ones who sleep, Galland et al. (2012), systematically reviewed children’ sleep patterns from birth to 12 years of age in a meta-analysis of 34 international studies. One-third of their studies were from non-Western countries (Saudi Arabia, Japan, Taiwan, Hong Kong, Korea, Russia, and China) because infants in Asian countries are reported to sleep less overall than infants in Western countries (Tham et al., 2017). Galland et al. (2012) found a wide range of normal sleep times across the reviewed studies, reflecting much variation in how long infants sleep in different samples. This variability is masked by the mean values they identified as international sleep norms for children (including nighttime sleep and naps): from 0 to 2 months, infants averaged 14.6 hours of sleep per day (range 9–20 hours); by 3 months, 13.6 hours (range 9–17 hours); by 6 months, 12.9 hours (range 9–17 hours); by 9 months, 12.6 hours (range 9–16 hours); by 12 months, 12.9 hours (range 10–16 hours); and by 1–2 years, 12.6 hours (range 10–15 hours). The longest sleep period was 5.7 hours for 0 to 5 months; 8.3 hours for 6 to 24 months. Night waking was common, averaging 1.7 times per night for 0 to 2 months; 0.8 for 3 to 6 months; 1.1 for 7 to 11 months; and 0.7 for 1 to 2 years. Notice a slight decrease in total sleep time around nine months of age and a slight increase in night waking at that same age—this well-known effect reflects the age when attachment needs coincide with the development of separation anxiety (see “Attachment” subsection).

Finally, as alluded to earlier, understanding infant sleep (and adult sleep) is important for the field of psychology as well as to the field of medicine/pediatrics. However, most studies of infant sleep published in psychology journals use techniques and assumptions from the medical approach (e.g., Hairston et al., 2016; Mindell et al., 2017; Teti et al., 2016). For a discussion decrying the lack of consensus on what “sleep quality” means for infants and the need for a new understanding of infant sleep, see Bernier et al. (2010).

Breastfeeding and infant sleep architecture

Currently, emphasis on infant nutrition in the West has returned to breastfeeding. The nutritional and developmental superiority of human breastmilk over artificial milk as the optimal, ideal food for human infants is unchallenged. In fact, every major health policy entity endorses exclusive breastfeeding for at least six months followed by continued breastfeeding for up to two years, including the World Health Organization (WHO, 2014), the American Academy of Pediatrics (AAP, 2019), the U.S. Department of Health and Human Services (2011), the U.K. National Health Service (2019), and many others. Worldwide increases in breastfeeding rates now require looking at “normal” infant sleep again because breastfed infants sleep differently than formula-fed infants, the subjects of 20th-century research.

Infant feeding method affects infant sleep (Russell et al., 2013), but research findings are inconclusive because studies use very different ages with different results. Some studies find that formula-fed infants sleep more than breastfed infants (Huang et al., 2015), while others show infants who breastfeed do not necessarily get less sleep than formula-fed infants (Doan et al., 2007; Engler et al., 2012; Pennestri et al., 2018; Ramamurthy et al., 2012), even if they wake more (DeLeon & Karraker, 2007; Galbally et al., 2013; Mindell et al., 2012; Weinraub et al., 2012). Still others report no differences in night waking between breastfed and formula-fed infants after six months of age (e.g., Brown & Harries, 2015). Other studies have shown changing differences in total sleep (Figueiredo et al., 2017) between breastfed and formula-fed infants under six months of age. However, formula-feeding mothers may overestimate their infants’ sleep when using maternal reports. When measured with a sleep actigraph, their infants’ sleep was not different from infants who were breastfed or who coslept (Rudzik et al., 2018).

Another reason feeding type is so important to understanding infant sleep is due to its effect on infants’ brain development. Neurodevelopment of formula-fed and breastfed infants is different; Jing et al. (2010, p. 123) stated, “brain maturation is guided by processes that are regulated and sequenced to be developmentally advantageous.” For example, being breastfed affected children’s academic test scores nine years later, over and above the effects of many possible confounds (McCrory & Layte, 2011). In addition, white matter development was increased in infants who were breastfed, especially in frontal brain areas that are associated with enhanced cognitive and behavioral performance (Deoni et al., 2013). While these studies do not address “normal” infant sleep directly, they help inform a better understanding of breastfeeding as an experience-expectant neurodevelopmental context for human infants’ brain evolution. Similarly, cosleeping could be conceptualized as an experience-expectant neurodevelopmental context (see later section on early sleep consolidation).

Cosleeping and infant sleep architecture

Sleep location (solitary vs. cosleeping) also influences how infants sleep, and with rising rates of cosleeping in the West (Colson et al., 2013), needs to be considered in understanding “normal” infant sleep. The term “cosleeping” is used in this paper to refer to mothers and infants sharing the same sleeping surface and in body contact for at least part of the night (Lozoff et al., 1996), following Huang et al.’s (2010, p. 170) definition: “an adult caregiver … sleeps close enough to her infant so that the mother and infant can respond to each other’s sensory signals and cues … [this] does not refer to simply sharing the same room.” The term cosleeping is preferred to the term “bedsharing” because it is general, historically accurate, and in accordance with the multidisciplinary nature of this paper, includes human evolutionary practices through human history long before there were beds.

Like the research on breastfeeding, cosleeping research is inconsistent in showing whether it increases or decreases infants’ sleep. Some studies show that cosleeping infants receive less sleep than solitary-sleeping infants (DeLeon & Karraker, 2007; Huang et al., 2015; Keller & Goldberg, 2004), while others show no differences by sleep location (Buckley et al., 2002; Mao et al., 2004). Buckley et al. (2002) particularly noted that total sleep time in full-term infants was highly regulated and relatively independent of specific child care practices. Although cosleeping mothers often report that their infants wake more at night compared with solitary sleeping infants (e.g., Teti et al., 2016), other work using sleep actigraphy rather than maternal reports has shown that cosleeping infants wake the same amount, on average, as solitary-sleeping infants but cosleeping mothers are more aware of their infants’ waking (e.g., Sadeh, 1996; Volkovich et al., 2015). This finding mirrors Rudzik et al.’s (2018) work with maternal reports of breastfed infants’ waking versus actigraphy. Similarly, cosleeping mothers are also more likely to do any breastfeeding and breastfeed for longer duration (c.f., Bovbjerg et al., 2018; Smith et al., 2016) and are more likely to breastfeed during the night (Buswell & Spatz, 2007; Little et al., 2018; McKenna et al., 1997; Russell et al., 2013).

In Western, industrialized nations where cosleeping is increasing after over a 100 years of decline, the biggest health concern is sudden infant death syndrome (SIDS). Health policy experts in the United States strongly recommend against any cosleeping because it is associated with higher rates of SIDS (American Academy of Pediatrics Task Force on Sudden Infant Death Syndrome, 2011). Other researchers disagree, finding that in the absence of other risk factors (i.e., smoking, formula-feeding, unsafe sleep surface), SIDS rates are no higher for cosleeping infants than for other infants (e.g., Blair et al., 2009, 2014; Mitchell et al., 2017). A review of SIDS rates compared with cosleeping rates around the world demonstrated that in many countries and cultures, cosleeping infants are less likely to die than solitary sleeping infants, especially of SIDS (McKenna & McDade, 2005). For example, SIDS rates are very low in China, even though cosleeping is very common there. One study of cosleeping Asian infants showed comparatively lower rates of SIDS, even in a population of infants with increased physiological risk factors for SIDS (Huang et al., 2010). A discussion over the relative benefits and risks of cosleeping is beyond the scope of this paper (but see Baddock et al., 2019; Bergman, 2013; Blair et al., 2014; Carpenter et al., 2013; Colvin et al., 2014; Gessner & Porter, 2006; Pelayo et al., 2006 for discussions of the issues), as is a discussion about why rates of cosleeping are rising (but see Barry, 2017 for a quick overview). Nonetheless, given the rising rates of cosleeping in the West and its ubiquity in the rest of the world, cosleeping must be included in any understanding of “normal” infant sleep.

Cross-cultural work on infant sleep around the world

“Co-sleeping is the normal, accepted pattern of night-time behavior in most cultures of the world today, as it has been throughout human evolution. … it’s just what people do” (Dettwyler, 1997, p. 160). For a “heat map” of cosleeping rates around the world based on a review of 659 papers, see Mileva-Seitz et al. (2017). Today, it is estimated that over 70% of the world’s peoples still engage in this ancient human practice (see Huang et al., 2010 for more details on cultural sleeping practices and social sleep around the world). As one author stated, “the modern Western custom of an independent childhood sleeping pattern is unique and exceedingly rare among contemporary and past world cultures” (Crawford, 1994, p. 46). The rise of cosleeping in the United States and other WEIRD nations over the past 30 years (Barajas et al., 2011) make an examination of “normal” infant sleep architecture and development a priority to better align research with human experience around the world today.

Cross-cultural studies highlight important differences in how cultural beliefs and values influence infant sleeping practices. For example, U.S. culture is highly independent and individualistic (Kitayama et al., 2010), making children’s “independence” one of the most important goals for parents (Keller & Goldberg, 2004). In the United States and other WEIRD nations, infants are believed to be born dependent with the need to develop independence as early as possible (beginning in infancy), in preparation for later life (Morelli et al., 1992). In other cultures, infants are believed to be born independent and must be socialized within the family to interdependence (McKenna et al., 2007). For example, Japanese parents view small children sleeping alone as rather “merciless,” while Mayan mothers equate solitary sleeping for infants as a form of child neglect (Morelli et al., 1992). In China, parents view solitary sleeping for infants as “unkind,” considering the sensory-rich nighttime cosleeping environment important for infant security, family intimacy, and long-term emotional development (Huang et al., 2010).

Parental attitudes about infant sleep

Societal expectations influence how adults think about infant sleep and are largely influenced by country/culture of residence and by feeding and sleeping practices. For example, New Zealand parents of children younger than two years expected them to be STN from about 8 p.m. to 6:30 a.m. (almost 10 hours of sustained sleep, twice as long as Moore and Ucko’s (1957) original definition; Henderson et al., 2013). In this study, presumably, mothers of 1-month-old infants as well as mothers of 18-month-old infants expected their babies to sleep almost 10 hours in a single stretch. In contrast, two-thirds of Canadian couples with six-month-old infants felt babies should not STN by six months (Kenny et al., 2019), with parents who were less favorable of cosleeping and whose babies were formula-fed being more likely to say their babies should STN by six months.

Another focus of a sociocultural view of parenting is managing the “work” of nighttime care of infants in the “forgotten third” of parenting—sleep (Riegle, 2017). Because parenting is a full-time job that occurs around the clock, pressures can mount on parents to encourage their infants to sleep through the night, so that they themselves can sleep through the night. As discussed previously in this paper, people in most of the world’s cultures do not face this conflict because their cultural expectations and practices more closely align with infant’s immature biophysiological development. In WEIRD countries, when parents’ and infants’ needs conflict, trade-offs must be made to balance infant sleep and nighttime care with parents’ need for sleep and self-care. Cosleeping is not culturally normative in the United States—not surprising in a culture where cosleeping is stigmatized and health policy experts use terms like “preventing cosleeping” or “contributing causes of cosleeping” to infer that cosleeping is a problem to be solved (Dettwyler, 1997). Thus, in the United States, many parents who ascribe to the cultural standard of solitary sleeping but end up cosleeping “slid into it as opposed to intentionally setting out to co-sleep” (Riegle, 2017, p. F18). In other words, in their search for more sleep to deal with their infant’s night waking, they happened upon the age-old custom of bringing the baby to bed with them (Tomori, 2014).

Ball et al. (2019) discuss this phenomenon in terms of how parents choose to allocate their maternal investment, all of which involve some sort of risk. Some mothers (typically adolescent mothers) reduce nighttime investment by falling asleep on a sofa, bottle propping with pillows, introducing solids early, and other behaviors. Other mothers (often older mothers) reduce nighttime investment by using soft toys, covers, and using separate rooms and sleep surfaces for infants (Volpe et al., 2013). The AAP notes that the risk of SIDS is reduced by 50% when the infant shares a room with the parent (Knopf, 2016)—that means that the risk of SIDS doubles when the infant sleeps alone in its own room. Still other mothers, often those who are breastfeeding, manage nighttime care by bringing the baby to bed for cosleeping and embracing shared sleeping, providing the infant and themselves with more sleep (McKenna et al., 2007; McKenna & Volpe, 2007; Quillin & Glenn, 2004). Bartick et al. (2018) question whether cosleeping represents a subordination of parental needs to infants’ needs (as has been suggested from a cultural viewpoint), pointing out that many breastsleeping mothers do not view it this way—breastsleeping is not necessarily a greater burden, as mothers may get different (but not less) sleep, and many do not mind it.

Social-structural factors influence infant sleep

Within cultures, different factors can influence family decisions around infant sleep, such as minority status, education, and socioeconomic status (SES). Within any given country, race and/or ethnicity that differs from the majority culture confers minority status. Members of minority groups commonly cosleep, especially in WEIRD countries. In the United States, Black families are more likely than White families to cosleep, regardless of SES (Colson et al., 2013; Lozoff et al., 1996; Salm Ward & Ngui, 2015; Willinger et al., 2003), and Hispanic mothers in New York were more likely than White mothers to cosleep (Medoff & Schaefer, 1993). In Canada, First Nation mothers are more likely to cosleep than White mothers (Bartick & Tomori, 2019). Similarly, in the United Kingdom, Pakistani and Bangladeshi families of any SES were much more likely to cosleep than White British families (Ball, 2012; Gantley et al., 1993; Tomalski et al., 2016), and in the Netherlands, Carribean mothers were more likely to cosleep than Dutch mothers (Luijk et al., 2013). In Thailand, minority Christian and Muslim mothers were more likely than Buddhist mothers to cosleep (Anuntaseree et al., 2008), and in New Zealand, Maori mothers have high rates of cosleeping (Bartick & Tomori, 2019; Rigda et al., 2000).

Level of education is associated with cosleeping around the world but in different ways. In the Netherlands, education and age were positively correlated with cosleeping rates (Tollenaar et al., 2012); highly educated mothers were more likely to engage in early cosleeping (intentional; based on an intent and belief that shared sleep is helpful) and mothers with low education were more likely to engage in late cosleeping (reactive; based on a reaction to perceived sleep problems and often done reluctantly; Luijk et al., 2013). In Thailand, older mothers with high levels of education were associated with higher cosleeping rates (Anuntaseree et al., 2008). Adolescent mothers in the United States have high rates of cosleeping (Caraballo et al., 2016), and cosleeping rates were higher for White mothers with low education (Salm Ward & Ngui, 2015), but other work has shown high cosleeping rates in highly educated, high-SES samples (Green & Groves, 2008). In the United States, many studies report correlations between cosleeping and low levels of education, but most are confounded with SES. Both low SES and low levels of maternal education are associated with cosleeping in the United States (e.g., Chianese et al., 2009), enhancing the difficulties of finding the independent contributions of those variables to cosleeping rates. Among White families only, lower SES was associated with more cosleeping (Colson et al., 2013; Lozoff et al., 1996; Willinger et al., 2003). Families with private health insurance (indicating higher SES) were less likely to cosleep than families without (Norton & Grellner, 2011).

Around the world, many sudden unexpected infant deaths (SUID) occur in marginalized populations in rich countries (Bartick & Tomori, 2019). Although cosleeping is high in these populations, it cannot be the main reason for these SUIDs because countries with low SUIDs have high cosleeping rates (c.f., Baddock et al., 2019; Bartick & Tomori, 2019). Given the low SUID rates with cosleeping in other cultures (especially across Asia), it is convenient but irresponsible to claim it is cosleeping and not other factors that are responsible. Rather, it is the associated behaviors (smoking, prone sleep, soft bedding, formula feeding, substance use, and impaired sleeping) that co-occurs in these populations that are closely linked to SUIDs (Baddock et al., 2019; Bartick & Tomori, 2019; Bergman, 2013; Blair et al., 2014; Gessner & Porter, 2006). In these groups, multiple risk factors combine to make cosleeping less safe for some groups but not others, leading to the conclusion that it is not cosleeping per se that is dangerous, but some of the conditions in which it occurs. Many of these variables are also correlated with sofa-sharing, a uniquely unsafe form of cosleeping which is a distinct practice from bedsharing with very little overlap (Ball et al., 2012). Thus, they should be considered separately in studies of the risks/benefits of cosleeping.

Chronic stress in marginalized population creates real health disparities, including pregnancy and birth outcomes and infant health. Such health disparities worsen some outcomes and are also related to SUIDs. Bartick and Tomori (2019) compared countries with high and low SUID rates and their prevalence of marginalized peoples, finding a relationship. “Factors that worsen income inequality, poverty, and racial marginalization can be expected to increase infant mortality” (Bartick & Tomori, 2019, p. 10). In 2014, the United States had highest SUID rate of developed countries. In fact, each of the social and cultural changes in infant sleep that have occurred in the United States over the past century are independent risk factors for SIDS or SUID (McKenna, 2014): shifting from cosleeping to solitary sleep, from breastmilk to formula, from back-sleeping that is a part of the breastfeeding complex to prone sleeping (which has shifted again since the “Back to Sleep” campaign of the 1990s), and promoting deep sleep and early sleep consolidation.

What is “sleep consolidation”?

A review of sleep consolidation (Henderson et al., 2011) found three relevant concepts that together define it: (1) longest sleep period, (2) longest self-regulated sleep period, and (3) STN. Each of these concepts has been studied in relation to infants’ sleep under the assumption that deeper, longer, solitary sleep is better. However, each of these components of infant sleep consolidation is problematic from other points of view under different assumptions. For example, see McKenna and McDade (2005) and McKenna and Mosko (1994) for discussions of how long sleep periods put infants at higher risk of SIDS and how cosleeping infants demonstrate greater physiological reactivity than solitary sleepers. Similarly, the longest self-regulated sleep period as a component of sleep consolidation is problematic. The most common definition is when the infant returns to sleep after waking during the night without signaling, or in other words, “self-soothes” (Henderson et al., 2011). While the goal of many sleep experts is self-soothing as early as possible, it apparently takes a long time to develop—half of 12-month-old infants in one study were not yet classified as self-soothers (Goodlin-Jones et al., 2001). Perhaps it is not surprising then, that early self-soothing has not shown to be related to later independence and no studies to date have linked infant “independence” with independence at later ages.

STN is also problematic as a marker of sleep consolidation. Despite researchers’ efforts to change the definition of STN to one that is more developmentally and socially valid (e.g., Henderson et al., 2011; Pennestri et al., 2018; Yoshida et al., 2015), it remains defined by individual researcher’s preference, cultural bias, or parental expectation, making STN an unreliable measure of sleep consolidation. For example, a review of infant sleep found that 20% to 30% of infants continued night waking through two years of age (Tham et al., 2017), and none of the studies showed consistent findings of sleep duration, sleep efficiency, night waking, or other sleep components benefiting cognition. Our understanding of where and when sleep consolidation should occur and definitions of “normal infant sleep” to date lack a biophysiological foundation and are largely culturally defined.

Finally, half the studies in Henderson et al.’s (2011) sleep consolidation review were from the 1990s or earlier, when fewer than half of infants were any breastfed and less than 20% of infants were exclusively breastfed for six months (U.S. Department of Health and Human Services, 2011). Their own research study indicated that only half of their sample were STN by five months, and some infants never reached criterion for STN by one year of age (Henderson et al., 2013). A logical conclusion is that even in solitary-sleeping infants, the 50% criterion suggests that sleep consolidation takes a long period of time (at least five months to a year), demonstrating that definitions of STN remain arbitrary, as it was just as “normal” not to reach criterion as to reach it. Therefore, their conclusions are highly influenced by Western sleep practices, out of line with normal infant sleep physiology, and do not reflect our nascent understanding of infant brain development as influenced by sleep environments.

Is early sleep consolidation “optimal” sleep?

As discussed throughout this paper,

there is no universally agreed-upon definition of what constitutes optimal sleep for preterm or full-term infants, “good” sleep for young children is generally characterized by independent sleep onset, longer consolidated sleep periods, self-soothing at night, and more sleep per sleep–wake cycle. (Schwichtenberg et al., 2013, p. 37; see also Burnham et al., 2002; Goodlin-Jones et al., 2001)

Considering infants’ immature brain development may help create a better understanding of the pattern of nighttime sleep consolidation in infants. From an evolutionary view, human infants are born with the most immature brain of any primate. Most human infants’ brain growth occurs after birth—it takes three years for our brain to reach just half the adult size. Konner (2010) suggested thinking of infant postnatal brain development as “postnatal neuroembryology,” demonstrating the utility of considering it as a seamless part of the developmental process. Such a view prompts researchers such as Jing et al. (2010, p. 123) to take as their assumption, “the working premise will be that the patterns of brain activity shown by breastfed infants reflect processes designed to best promote Central Nervous System (CNS) development.” Here, breastfeeding represents a species-typical environment that represents the feeding experience of millions of years of human evolution. The long period of brain development for human infants evolved in the context of human breastmilk as a nutritional source and cosleeping as a developmental microenvironment. Might cosleeping represent the sleep environment designed to best promote CNS development?

Just as infant “independence” might look different than child or adult independence, it is possible that what is preferable for adult sleep is different for infants. “Normal” infant sleep may include cosleeping as a species-typical environment for postnatal neuroembryological growth. For example, EEG changes in infant sleep are known to continue changing well beyond six months (e.g., Novelli et al., 2016), so that is not a good cutoff for identifying “adult-like” EEG activity. Similarly, the amount of sleep for a one year old is still more than 50% of the day, which is not adult-like, so why should experts expect infant sleep be “adult-like” in other ways? Brain activity is difficult to study in infants, due to the challenges of obtaining accurate EEG measurements and interpreting them (REM sleep during infancy especially resembles and is hard to distinguish from wakefulness; Andrillon et al., 2011; Eisermann et al., 2013; Mitra et al., 2017; Yoshida et al., 2015). Nonetheless, many studies have used the technology of EEG recordings to describe “normal” infant sleep. Most infant EEG studies focus on active and quiet (SWS) sleep, concluding that adult-like EEG patterns emerge between six and nine months with clearly identifiable SWS and REM sleep, marking a turning point for sleep maturation (Louis et al., 1997). As noted earlier, though, much of this work is based on formula-fed, solitary-sleeping infants, which do not represent the environments for brain development in most of the world’s infants.

In a study investigating SWS in Stages 3 and 4 sleep in breastfed infants from ages two weeks to nine months (cosleeping status unknown), Jenni et al. (2004) found decreasing active sleep and increasing quiet sleep with age, confirming other studies. In addition, however, they evaluated the specific EEG sleep bands that make up infant SWS, concluding that neurological processes may serve different functions in adult and infant sleep. This is especially probable given the infant’s immature neurophysiological development and highly plastic brain. Sustained slow-wave delta bands on EEG (defining SWS in adults) do not appear until age two to three years (Mitra et al., 2017). While consolidated nighttime sleep may be “good” for adults (whether it is “good” or “normal” is also a matter of some debate but is beyond the scope of this paper, see Ekirch, 2005; Roach et al., 2017; Wehr, 1992 for relevant discussions) that does not necessarily support the assumption that early consolidated nighttime sleep is “good” for infants.

More recent EEG work focuses on the development of sleep spindles, which occur during non-REM sleep and play a functional role in cortical development, learning mechanisms, and memory consolidation (Andrillon et al., 2011; D’Atri et al., 2018). Changes that occur during infancy provide a window into neurodevelopmental plasticity, with the first mature form of sleep spindles appearing at about 12 months of age (D’Atri et al., 2018). Similarly, emerging functional magnetic resonance imaging (fMRI) studies is are also beginning to elucidate differences in infant and adult sleep. For example, Mitra et al. (2017) found fMRI of sleeping six month olds to most closely resemble that of awake adults. This effect diminishes over time, however, such that two-year-old fMRI more closely resembles the corresponding adult states. As postnatal brain development in human infants is highly ordered and sequenced, reflected in changes of metabolic activity, myelination and gray matter maturation (Mitra et al., 2017), is it possible that early sleep consolidation is not optimal, and that night waking serves a role in infant neurodevelopment?

Nutrition

Probably the most widely known reason for night waking in early infancy is to satisfy infants’ need for nutrition as human infants have small stomachs and will wake from sleep to meet their need for food. Breastfed babies awake more often and are more easily arousable than babies fed artificial formula, and this is likely one reason that breastfed babies are at lower risk of SIDS than babies fed formula (Bartick et al., 2018; McKenna & McDade, 2005). However, providing formula to reduce night waking has not been shown to improve infant sleep (Doan et al., 2007; Montgomery-Downs et al., 2010). Throughout human history, “the availability of breast milk has always been … the single most important predictor of infant survival” (Hrdy, 1999, p. 301). The physiological and bioactive content of human breastmilk is fundamentally different from cow’s milk, which was originally the basis of all artificial formulas (Bartick et al., 2018). The composition of human breastmilk (low protein and fat) as a low solute milk requires frequent feeding, low suck rates, and close proximity of mother and infant both day and night (Ball, 2003; Barr, 1999). Compared with cow’s milk, human breastmilk has less than one-third proteins, and the same amount of fats and casomorphins, which have soporiphic properties. However, cow’s milk is more likely to lead to deeper sleep in infants (and therefore fewer waking) partly because the high concentration of caseins in cow’s milk solidifies in the stomachs of infants fed formula, delaying gastric emptying (McKenna, 2014; casein is what causes milk to solidify into cheese in the presence of enzymatic action).

In addition, human breastmilk has higher lactose content compared with cow’s milk, thus increasing gastric motility in breastfed infants, resulting in quicker and more thorough digestion of breastmilk than formula (McKenna, 2014), requiring more frequent feeding. In the first 8 to 12 weeks of life, human infants must wake every 2 to 3 hours to nurse in order to maintain their rapid growth rate of body and brain (Ball & Russell, 2012; Bartick et al., 2018). Frequent feeding, especially at night, also helps nursing mothers to establish and maintain their milk supply. Prolactin, which increases human milk production, is in its highest concentration in breastmilk during the night, encouraging nighttime feedings (Cregan et al., 2002). Without nighttime feedings, milk production is reduced, which can result in breastfeeding cessation for some women. Another reason for breastfeeding cessation is unidentified breastfeeding problems, such as an unsettled baby’s mother believing she does not have enough milk and initiating formula feeding (Bartick et al., 2018) or functional lactose overload leading to excessive and painful infant gas (Douglas & Hill, 2013). Unidentified and unmanaged difficulties in feeding can also lead to increased night waking and signaling (Douglas & Hill, 2013).

Mother–infant synchrony

Some infant night waking is due to mother–infant neuroendocrine and neurobehavioral synchrony, especially for cosleeping and breastfeeding infants. This mother–infant synchrony is biologically programmed but subject to maternal social behaviors (Douglas & Hill, 2013; Feldman, 2007; McKenna & Mosko, 1994). Synchrony is created by consistent, frequent, repeated, and predictable actions by parents. These actions set up mutual and synchronous behavioral exchanges between the caregiver and infant. These are so important in establishing synchrony because it is the parental behavior (and not parental intentions or beliefs) that is directly available to the infant’s perceptual experience (Feldman, 2012). Parent–infant physiological and biobehavioral synchrony establish the infant’s biological organization and emerging, eventually independent, psychosocial regulation. Nighttime caretaking offers prime opportunities for frequent, consistent, repeated, and predictable parental actions such as responding to infant night waking. For example, infants cried less when their mothers responded more quickly and had shorter intervals between feedings (Barr & Elias, 1988). Especially when combined with breastfeeding, cosleeping both increases nighttime feeds for breastfeeding mothers and allows for immediate responsivity, both of which serve to reduce crying.

Infant and maternal care cannot be disentangled at the earliest stages, suggesting that the human fetus is “presensitized” to sensory cues from the mother (movement, touch, temperature, gas exchanges, etc.) which continue after birth (Winnicot, 1965). After birth, the completely biologically dependent human infant adapts to the external environment, eventually making the transition to biological independence, in part through continued maternal–infant sensory exchanges (Winberg, 2005). For example, Mosko et al. (1997) identified that carbon dioxide (CO₂) exchange between mothers and cosleeping infants was at very low levels. Rather than acting as a risk factor for SIDS at these levels, it served to stimulate infant breathing and actually increased breathing stability and healthy infant respiratory development. Such sensory exchanges are part of parent–infant biobehavioral synchrony, which must be established in the newborn period to support neurobehavioral maturation, stress regulation, and future socialization experiences (Feldman, 2012). When the mother reads, responds to, and/or shares the infant’s physiological and/or biobehavioral states (Feldman, 2007) through a process of mutual regulation, the result is maternal–infant synchrony.

The cosleeping microenvironment may particularly promote and support mother–infant mutual synchrony. For example, McKenna and Mosko (1994) found that mothers and infants shared mutual arousals during the nights spent sleeping together, where mothers aroused first between 10% and 25% of the time and infants aroused first over 50% of the time. These mutual arousals increase parental sensitivity by increasing mother–infant synchrony and responsivity. Research on the biological foundations of parent–infant synchrony and developmental outcomes (e.g., Feldman, 2007) shows the importance of elements such as touch, arousal, proximity, body movement, and body position in developing synchrony during early infancy. Each of those elements is particularly activated in the cosleeping microenvironment. McKenna and Mosko (1994) found that mothers and infants had more arousals and had synchronous arousals with the infant when they coslept compared to when they slept alone. Mothers then responded faster to checking on the infant and tending to the infant’s nighttime needs. Thus, cosleeping provides additional opportunities for establishing parent–infant synchrony, continued into nighttime care. Research has also found that early parent–infant synchrony is related to better cognitive, social, self-regulatory, and attachment outcomes across childhood (Mileva-Seitz et al., 2017).

Another form of mother–infant synchrony is referred to as the “Complex Adaptive System (CAS)” by Douglas et al. (2011, p. 794):

The mother–baby CAS spontaneously self-organises according to multiple feedback loops—biochemical, neurohormonal and behavioural—within the infant, within the mother, between mother and infant, and within the broader familial and sociocultural systems. Because of self-organisation, CASs show resilience in the face of perturbation, and are characterized by a high degree of flexibility and adaptivity. As a result, the human infant is extremely adaptive to a wide range of infant-care norms across diverse cultures.

Temperament

Infant temperament has also been implicated in infant sleep and night waking (Ednick et al., 2009; Hairston et al., 2016; Scher et al., 1998). Specifically, infants with difficult temperaments are more likely to wake at night and sleep less (see also Weinraub et al., 2012). Infants with easy temperament, more approachability, rhythmicity, adaptability, and low distractibility received increased sleep compared with other temperament types (Spruyt et al., 2007). However, infants who were more rhythmic and predictable sleepers also had more fragmented and less efficient sleep (Scher et al., 1998). An analysis of infant surgency (positive emotionality with verbal and physical expressiveness and novelty approach) found that highly surgent infants gained sleep over the first six months of life when their mothers were emotionally available at bedtime (Jian & Teti, 2016).

Maternal depression

Numerous studies in past decades have found an association between infant sleep problems (usually defined as night waking) and maternal depression. Some authors suggest this is due more to the parents’ mental health than with the actual quality of infant sleep. For example, income and stressful life events moderated the relationship between maternal mental health and infant sleep quality (Goldberg et al., 2013). Mothers with low SES and high stress were most likely to suffer negative mental health consequences due to bedtime struggles or nighttime difficulties. Goldberg et al. suggested that high stress could reduce parental sensitivity and responsiveness, which could decrease infant sleep quality, beginning a negative cycle which contributed to low-quality infant sleep and higher mental health problems (especially depression, anxiety).

There is a relationship between mothers’ depressive symptoms and infant night waking (Bhati & Richards, 2015; Gress-Smith et al., 2012), where maternal sleep quality mediates the effect of maternal depression on infant night waking. In other words, when maternal sleep quality declines due to depression, infants wake more during the night (Hairston et al., 2016), partially due to maternal behaviors during nighttime caretaking. In addition, infant sleep architecture in the first six months of life is significantly altered in the context of maternal depression at the time of birth, suggesting that prenatal environmental exposure to maternal depression could affect the infant’s neurodevelopmental plasticity (Bat-Pitault et al., 2017). Is it possible that more night waking by infants of depressed mothers reflect an infant’s bid to build attachment? Secure attachment is less likely in the presence of maternal depression (Field, 2017; Hairston et al., 2016; and see C. Martins & Gaffan, 2000, for a review).

Attachment

Synchrony is supported by the physiological systems that also support attachment development (Feldman, 2007). Thus, night waking may serve as an affiliative bridge between the infant and his/her mother, encouraging attachment. Attachment refers to the reciprocal, enduring emotional bond between caregiver and infant that sets the infant’s expectations for future human relationships (Berk, 2015). Parenting systems are active during the nighttime as well as during the daytime, making nighttime caretaking another opportunity for the development of caregiver–infant attachment. According to Bowlby (1988), separations of mothers and their infants during the night are stressful, activating the attachment system. Therefore, night waking represents an opportunity for reunion and comfort (Goodlin-Jones et al., 1997). Caregiver–infant attachments are measured at 12 months of age and are categorized as secure (two-thirds of U.S. infants) or insecure (one of the three types). In secure attachment, the infant seems confident in the caregiver’s ability and availability. The infant shows alarm in the presence of a stranger or the absence of the caregiver, but when the caregiver is present, feels free to explore a new environment. Decades of research shows this type of attachment to have the best outcomes in infants’ physical, social, emotional, and cognitive development, both in the short term and long term (Berk, 2015). In insecure attachment (three types), infants seem to lack confidence in the caregiver’s ability and/or availability and respond with clingy, resistant, avoidant, or unpredictable behaviors.

Quality nighttime caretaking and availability (including sensitivity, appropriateness, consistency, and warmth, such in the cosleeping microenvironment) are related to attachment security in infants. For example, Mileva-Seitz et al. (2016) found solitary sleepers to be at significantly greater risk of having insecure-resistant attachment than cosleepers (defined as any cosleeping). This means that infants who were encouraged to be independent were expected to self-soothe from birth and to fall back asleep on their own upon night waking were more likely to exhibit “clingy” behavior at age one year. While responding to daytime crying is generally viewed as good childcaring practice, responding to nighttime crying is often assumed to prevent the infant from learning to self-soothe and sleep through the night (Bruni et al., 2014; Goodlin-Jones et al., 2001; Karraker, 2008; St. James-Roberts et al., 2015). However, mothers who picked up and soothed their one-year-old old infants when they woke fussing/crying/signaling during the night rather than left them to cry had infants who were much more likely to be classified as securely attached (Higley & Dozier, 2009). Perhaps parental responses to daytime crying but not to nighttime crying confuse the infant, who may perceive that parental responding is inconsistent, affecting the attachment relationship and making secure attachment less likely (Blunden et al., 2011).

The relationship between night waking and attachment is not just unidirectional, however. Night waking patterns are related to attachment style. While infants had similar night waking across the first seven weeks of life, beyond that Beijers et al. (2011) found that infants who were later identified as insecure-avoidant woke significantly less often than infants later identified as securely attached. Infants later identified as insecure-resistant woke more often in the first six months. By the age of one year, infants classified as insecure-avoidant did not need resettling if they woke, but the other groups still needed help resettling when they woke once or twice per week. Regardless of attachment style, as the infant approaches the age of nine months, separation anxiety appears. The infant prefers the presence of the attachment figure over other people, actively seeking him/her out and showing anxiety in his/her absence (Bowlby, 1988). These attachment needs are activated at night as well as during the day, resulting in increased night waking around this time and increased nighttime care. Several studies show increases in night waking at nine months of age (c.f., DeLeon & Karraker, 2007; Galland et al., 2012), decreasing again by one year of age.

Consistent cosleeping during the separation anxiety phase reduced sleep fragmentation relative to solitary sleepers (Teti et al., 2015). Prior to nine months, sleep fragmentation had been similar in cosleepers and solitary sleepers. This suggests that consistent cosleeping provides an environment where infants would be immediately reassured of parental presence upon waking, allowing them to fall back asleep readily, and getting more sleep, while solitary-sleeping infants would be more likely to wake longer or signal to parents for reassurance. Finally, McNamara (2004) ventured that REM sleep evolved in part to encourage social bonding between mammalian mothers and infants because REM sleep (in the context of cosleeping) increases synchronicity and activation of neuronal pathways that are integrated with the attachment system for social bonding.

Effect on interbirth interval

Another possible reason for infant night waking may be that nocturnal breastfeeding has contraceptive value for the mother as well as nutritional value for her infant (Haig, 2014). Short interbirth intervals increase mortality of both the older and younger child, and nighttime breastfeeding extends lactational amenorrhea when continued after seven months of age (Elias et al., 1986). Haig (2014) argued that human infants may have evolved to continue nighttime feedings as an aid to their own survival by delaying their mother’s next birth. In response, Wilkins (2014) suggested that an infant disrupting his own sleep to increase the interbirth interval would be counterproductive since it reduces how much sleep the infant receives and may even be maladaptive. Such a response, however, is dependent on the underlying assumption that night waking is bad and interferes with development. In another response, McKenna (2014) cautioned that there are many reasons infants wake at night, including nutrition, warmth, reassurance, mutual arousals which help create mother–infant synchrony, among others. It is possible that infants have evolved to have more arousals because more arousals lead to greater chances of survival. During nighttime care, infant arousals and maternal arousals both often result in nighttime feedings in breastfed infants, especially those who cosleep. Mothers may have similarly evolved to respond to nighttime arousals because breastfeeding is related to lower breast cancer and ovarian cancer rates, affecting their own survival. McKenna also mentioned that infants who are touched and cuddled by their mothers upon waking show transitional higher heart rates and greater blood oxygenation, which is good for infant growth and development.

Night waking to improve infant sleep?

Breastmilk is a bioactive, dynamic, and responsive food with complex adaptations for infant growth and development. Evolution has supplied human breastmilk with components that work to help develop the infant’s circadian rhythm and sleep-wake cycle. For example, melatonin has been suggested as the mechanism through which breastfeeding increases infants’ SWS (Yoshida et al., 2015)—melatonin concentrations in human breastmilk are low during the day but peak at 3 a.m. (Ball et al., 2019). Similarly, the circadian rhythm of tryptophan in human breastmilk peaks at 3 a.m., promoting infant sleep (Cubero et al., 2005). Adding to the endocrinological load to enhance infant sleep at night is the release of oxytocin with breastfeeding, which helps both the mother and infant sleep. Together, the evidence from endocrinology suggests that infants’ night waking is not an artifact resulting from night/day confusion but instead may occur for a different purpose—to help promote nighttime sleep even while maintaining optimal nutritional intake. Thus, circadian variations in the components of human breastmilk influence infant sleep consolidation into nighttime rest (Cubero et al., 2005) but within the context of night waking.

Lack of training

For many sleep experts, infant night waking occurs due to lack of training. In other words, infants have yet to be taught how to sleep through the night, which some feel is necessary and easily done to prevent sleep problems (Henderson et al., 2013). However, there are more studies showing that sleep training is effective than there are asking whether or not it is desirable. Most sleep training studies use total sleep time, number of night wakings, or sleep efficiency (i.e., spending a larger percentage of time asleep between sleep onset and wakefulness) as an outcome measure, rather than attachment, maternal–infant synchrony, or other long-term cognitive or social outcomes.

“For every complex problem, there is an answer that is clear, simple and wrong.” Attributed to American journalist H. L. Mencken, this quote highlights the risk of focusing on sleep training without understanding why infants wake at night. This is especially true if the phenomenon of interest (night waking in normally developing infants) is not actually a “problem.” For example, Price et al. (2012) published a study of seven-month-old infants who underwent a behavioral sleep training program and examined on a variety of behavioral measures five years later. The authors concluded “behavioral sleep techniques have no marked long-lasting effects (positive or negative)” (Price et al., 2012, p. 643). However, their conclusions have been roundly criticized for methodological and ethical concerns as well as how the conclusions were drawn (they did not test for many possible effects so cannot reasonably conclude there were no long-term negative effects; see comments to the published article at pediatrics.aappublications.org).

It is clear that some sleep training protocols result in behavioral changes in infant sleep, at least in the short term. However, the negative outcomes of sleep training infants need to be considered, especially if sleep training is unnecessary based on an understanding of “normal” infant sleep across the first year of life. For example, when mothers perceive infants’ night waking as nonnormative, they often interpret it to mean their breastmilk is inadequate in quantity or quality, which can end breastfeeding (Ball et al., 2019). Since breastmilk is universally accepted as the ideal food for optimal infant health and development (Little et al., 2018), anything that interferes with breastfeeding necessarily has long-term effects on infants’ physical and cognitive developmental outcomes. Other unintended negative outcomes include increased crying, increased maternal anxiety, possible effects on attachment outcomes, and increased risk of SIDS (for solitary sleepers). Sleep training may be effective after six months or one year but should not be prematurely applied (Douglas & Hill, 2013). There are major differences in neurodevelopment before and after six months of age, so using behavioral interventions during the first stage may not be warranted. “The belief that behavioral intervention for sleep in the first 6 months of life improves outcomes for mothers and babies is historically constructed, overlooks feeding problems, and biases interpretation of data” (Douglas & Hill, 2013, p. 497).

Some researchers have resorted to drastic measures in their quest for early sleep consolidation to end night waking. Despite the WHO’s (2014) recommendation of exclusive breastfeeding for six months before introducing solid foods, Perkin et al. (2018) introduced early solids (at three months). After three years, they found that sleep differences between the two groups peaked at six months, when the intervention group slept for 17 minutes longer than control group, and their night waking decreased from 2.01 times per night to 1.74 times per night. Responding to this study, Felder and Lee (2019) noted that the overall average sleep increase after intervention, three years later, was 7 minutes. They questioned whether that amount of sleep is meaningful for parents, especially when there was no measure of parental sleep gains or losses. It should be noted that both the AAP in the United States and the U.K. National Health Service in the United Kingdom, like the WHO, recommend at least six months of exclusive breastfeeding before introducing solids to infants. Parental concerns about night waking (and assumptions that night waking means infants cannot settle themselves) often lead to recommendations for sleep training. The most common methods of behavioral interventions include delayed parental response to infant signaling, introducing stricter feeding regulation, setting standards for sleep length and bedtimes that all require solitary sleeping, and making sure the infant is put to bed awake. Each of these except for putting the infant to bed awake has also been found to interfere with breastfeeding. Furthermore, cue-based care is important in infancy but is incompatible with behavioral training interventions designed to delay responding to infant cues (Douglas & Hill, 2013).

Recommendations for solitary sleeping (and against cosleeping) to improve infant sleep are like “throwing the baby out with the bathwater.” Cosleeping has many benefits to infants and mothers who choose it, but just because cosleeping is sometimes not done safely does not mean it can never be done safely. Rather than recommend against all cosleeping (as health policy experts in the United States do), parents should be provided information to make an informed decision and to make behavioral changes to safely support their decision (more like occurs in the United Kingdom; Ball, 2017). Despite the increases in recent years of infants and children being forgotten in car seats and dying in hot cars during hot weather in the United States, no one is calling for a ban on car seats or for parents to stop transporting children in cars. Other solutions are sought because of the cultural assumption that cars and car seats are indisputable parental needs, which means that parents must be provided information to help them avoid the possibility of infant harm. Some cars are now being manufactured with alarms that sense when a child is left in a car seat (Cleave, 2018).