Objective Daytime Napping is Associated with Disease Severity and Inflammation in Patients with Mild to Moderate Dementia 1

Abstract

Background:

Patients with dementia report excessive daytime sleep/sleepiness, which is associated with worse cognitive performance. Inflammatory markers may be elevated in patients with dementia and have been proposed as mediators of sleep/sleepiness.

Objective:

To examine the association of objective daytime napping with cognitive performance and peripheral markers of inflammation in patients with dementia as compared to not cognitively impaired (NCI) controls.

Methods:

A sub-sample of 46 patients with mild-to-moderate dementia and 85 NCI controls, were recruited from a large, population-based cohort of 3,140 elders (≥60 years) in Crete, Greece. All participants underwent medical history/physical examination, extensive neuropsychiatric and neuropsychological evaluation, 3-day 24 h actigraphy and a single morning measure of IL-6 and TNFα plasma levels. Comparisons of sleep parameters and inflammation markers between diagnostic groups, and between nappers and non-nappers within each diagnostic group, were conducted using ANCOVA controlling for demographics/related clinical factors. Associations between inflammatory markers, sleep variables, and neuropsychological performance were assessed within each group using partial correlation analysis controlling for confounders.

Results:

Patients with dementia slept 15 minutes longer during the day than NCI. Within dementia patients, nappers had significantly worse performance on autobiographic memory (p = 0.002), working memory (p = 0.007), episodic memory (p = 0.010), and assessment of daily function (p = 0.012) than non-nappers. Finally, IL-6 levels were significantly associated with nap duration within dementia patients who napped (r = 0.500, p = 0.01).

Conclusions:

Daytime napping in patients with dementia is associated with worse cognitive performance and increased IL-6 levels. In dementia, objective daytime napping, may be a marker of the severity of the disease.

Keywords

Actigraphy cognitive performance cytokines dementia inflammation objective daytime napping

INTRODUCTION

Dementia, based on the World Health Organization definition, is a syndrome presenting with deterioration in memory, thinking, behavior, and ability to perform everyday activities, leading to functional impairment [1]. Dementia is associated with increased comorbidity and institutionalization [2, 3] and increased personal, family and social burden, as indicated by several years of disability and high health care cost [4]. The most common type of dementia is Alzheimer’s disease (AD), while other common types include vascular, Lewy body, and frontotemporal dementias. In AD, memory impairment is the cardinal symptom, especially in the mild to moderate forms of the disease [1].

Many studies have evaluated sleep in patients with dementia. The most frequent objective sleep disturbances among patients with dementia are lighter, longer, and fragmented sleep [5 –7], as well as impairment of the basic circadian sleep-wake cycle rhythm [8]. It has further been suggested that excessive daytime sleep and sleepiness may be associated with worse cognitive function in patients with AD [5 , 9–11].

Recent studies have demonstrated that in neurodegenerative dementias, brain pathology can result in the activation of the inflammatory cascade [12 –14]. Although the role of inflammation in AD pathogenesis is not clear, pro-inflammatory cytokines, such as interleukin (IL)-1β, IL-6, and tumor necrosis factor α (TNFα), are possibly involved [15]. Specifically, elevation of IL-6 and TNFα have been detected in patients with AD [15 –17].

The pro-inflammatory cytokines IL-1β and TNFα are involved in the regulation of sleep both in animals and humans [18, 19]. Moreover, elevated cytokine levels have been linked to excessive sleep/sleepiness. It has been shown that exogenous administration of IL-1β and IL-6 to patients can induce somnolence and/or increased sleep [19]. Peripheral levels of IL-6 and TNFα are elevated and appear to mediate sleepiness and fatigue among patients with excessive daytime sleepiness disorders, such as sleep apnea and narcolepsy [20], while in a pilot study, Etanercept, a TNFα antagonist, significantly and markedly reduced daytime sleepiness in patients with sleep apnea [21].

Several studies have investigated the changes of inflammatory markers as well as the prevalence of sleep disturbances and sleepiness in patients with dementia. However, research focused on the association between inflammation and sleep in this group of patients is very limited [22]. The aim of the present study was to test the hypothesis that objective daytime sleep is associated with worse memory performance and informant-rated daily functionality, as well as increased pro-inflammatory cytokines IL-6 and TNFα in patients with dementia.

MATERIALS AND METHODS

Study design

The sample of the current study was selected among participants of the Cretan Aging Cohort (CAC), a cross-sectional study of community dwelling elders, recruited from the area of Heraklion, Crete, Greece, investigating prevalence and risk factors associated with cognitive impairment [23]. The study was approved by the Bioethics Committee of the University Hospital of Heraklion, Crete, Greece (protocol number: 13541, 20/11/2010). This study was conducted in two phases and is described in detail in a previous publication [23].

Phase I

Eligible participants were those aged ≥60 years who visited selected Primary Health Care (PHC) facilities in rural and urban areas of the Heraklion district for any reason. Consenting individuals (n = 3,200) completed an interview with a trained nurse, using a structured questionnaire to record sociodemographic information (obtained by the clinicians), anthropometric measurements, physical and mental health problems, and medication use. Cognitive function was evaluated using the Greek version of the Mini-Mental State Examination (MMSE) test [24], employing a universal cut-off of 23/24 points (since the majority of participants had ≤6 years of formal education) for referral of patients for further evaluation.

After excluding participants with crucial missing data i.e., MMSE, age, the final study sample consisted of 3,140 persons (57.0% women), aged 73.7±7.8 (60–100) years who had completed an average of 5.8±3.3 (0–18) years of formal education (82.7% and 23.9% with ≤6 years and ≤4 years of formal education, respectively) and lived mostly in rural areas (86.2%) (Fig. 1).

Fig.1

Study Flowchart. MMSE, Mini-Mental State Examination; NCI, non-cognitively impaired.

Phase II

Based on Phase I evaluation, all participants with MMSE < 24 (n = 636) were referred for neuropsychological and neuropsychiatric assessments. However, 344 participants were eventually evaluated with a response rate of 54.1% (45.9% attrition rate). Participants with low MMSE who were not evaluated for any reason (n = 292; refused, could not be located, passed away) did not differ from the 636 subjects referred, in terms of age, sex, and body mass index (BMI) (Fig. 1) [23]. For those participants who could not be located after three unsuccessful phone calls, an invitation letter was sent to their home address. The Phase II assessment involved administration of an extensive questionnaire (modified from the one employed in the HELIAD study [25]) on determinants of health status in the context of a semi-structured interview by a team of certified neurologists, psychiatrists, and internists. Neuropsychological assessment was performed by trained neuropsychologists using a test battery evaluating a variety of cognitive domains [23]. Dementia diagnosis, based on a thorough neuropsychiatric and neuropsychological assessment, was performed according to the DSM-IV criteria [26]. Probable AD, vascular dementia, Lewy body dementia, behavioral variant frontotemporal dementia, and other types of frontotemporal dementia were diagnosed according to the NINCDS-ADRDA [27], the NINDS-AIREN [28], the DLB Consortium [29], the International Consortium on behavioral variant frontotemporal dementia [30], and the Neary [31] criteria, respectively. Mixed dementia was diagnosed in cases of coexistence of both probable AD and vascular dementia in the same patient [32].

A comparison group, the size of which was set to approximately 30% of the group of participants with MMSE < 24 referred to Phase II, was formed by applying a proportional stratification procedure using residence (geographic region within the Heraklion prefecture), age, and sex as the stratification variables to the remaining 2504 persons who scored ≥24 points on MMSE. When the number of eligible participants in a given cell exceeded the targeted percentage (i.e., the corresponding percentage in the low MMSE group), random selection was applied to complete a final list of 181 persons to be invited to participate in Phase II of the study. Of those, 161 agreed to participate and were administered the neuropsychiatric and neuropsychological assessment (response rate 88.9%, attrition rate 11.1%). Based on these assessments, 105 persons were found to be cognitively non-impaired and 85 persons provided usable self-reported sleep, actigraphy, TNFα, and IL-6 plasma levels data [not cognitively impaired (NCI) group] (Fig. 1). This subgroup did not differ significantly from the larger group of 181 persons on age, sex, and BMI. The diagnosis of dementia was conveyed to the participants with a detailed letter to the primary care physician with instructions about follow-up care and possible treatment.

Participants

The current analyses included a subsample of participants from Phase II (n = 505) with a diagnosis of dementia (n = 46) or NCI (n = 85), who also had valid actigraphy, subjective sleep data, and measurement of pro-inflammatory cytokines (IL-6 and TNFα) (Fig. 1). Both NCI participants and dementia patients included in these analyses were not significantly different in terms of age, sex, and BMI compared to the total NCI and dementia group of Phase II, respectively. Given that all participants of this cohort were community-dwelling visitors of Primary Care Health units, elderly with severe terminal, medical illnesses or severe movement impairment were not included in this Cohort, and all patients had mild to moderate dementia (as indicated by MMSE score <12). Furthermore, none of the participants had a diagnosis of polymyalgia rheumatica or evidence of intercurrent infections during the time of evaluation.

Sleep measurements

Subjective sleep

Subjective sleep and presence of any sleep disorder were evaluated using a standardized questionnaire, described in detail elsewhere [33]. The presence of excessive daytime sleepiness (EDS) was indicated by a positive response (“often” or “always”) to one or both of the questions “Do you feel groggy or sleepy most of the day but manage to stay awake”, and/or “Do you have irresistible sleep attacks”. We also ascertained the presence of symptoms consistent with sleep disordered breathing indicated by a positive response on one or both of the following questions: “Do you know/Have you been told that you stop breathing or breath irregularly during sleep” and “Do you know/Have you been told that you snore during sleep” (as positive response were considered rating of occasionally/often/always on the former and moderate/severe degree on the latter question).

Objective sleep

Objective measurement of sleep was recorded for three consecutive 24 h periods through actigraphy using an actigraph (Actlife, GT3XP model, Pensacola, FL, USA). The device was placed on the non-dominant arm and the participant (or caregiver for dementia patients) was instructed to fill out sleep diaries reporting “bedtime”/“out of bedtime” and times the actigraph was removed on a daily basis. Night was defined based on the questions “what time did you go to bed” and “what time did you get out of bed” filled by the participant in the sleep diary. Naps were documented based on the actigraphy and the sleep diary provided by the participant or caregiver. A nap was defined as daytime sleep period longer than 20 min. Offline analysis of actigraphy data using the ActLife 6 software (Actlife v6.9.5 LLC, Pensacola, FL, USA) involved computation of the following nighttime, daytime, and 24 h sleep indices: nighttime sleep latency (nSL), nighttime total sleep time (nTST), daytime nap total sleep time (dTST), 24 h (nighttime and daytime) total sleep time (24 h TST), and 24 h (nighttime and daytime) time in bed (24 h TIB). Average sleep parameters over three consecutive 24 h in a free-living environment were estimated. Actigraphy recordings were conducted during weekdays for all participants. Participants with at least one nap during the three 24 h actigraphy were classified as “nappers”, while those without any nap were classified as “non-nappers”. The start time of the 24 h period was set at 11 : 00 a.m. on the day that the actigraph was applied on the participant’s hand by our staff. Indices of objective daytime napping were daytime nap TST, and daytime nap sleep latency. Participants with actigraphy recorded over fewer than 3 days or average TST≤3 h were excluded from the analysis.

Neuropsychological measures

Verbal working and episodic memory (word list retention capacity) were assessed using tests that have been standardized in the Greek population. Working memory was assessed using the Memory for Digits-Reverse and memory for personal events (autobiographic memory) through corresponding subtests from the Greek Memory Scale [34]; verbal episodic memory was assessed via the Rey Auditory Verbal Learning Test, retention index [35, 36]; and daily functional capacity was assessed using the 13-item Greek Index of Independence of Activities of Daily Living (IADL) scale [34], administered in the form of a structured interview to the participants’ closest relative or caregiver.

Inflammation markers

Single, morning, blood samples were collected between 10 : 00 a.m. and 12 : 00 p.m., transferred to EDTA-containing tubes (3 per patient) and refrigerated until centrifugation (within 3 h) for the plasma isolation which was kept in deep freeze (–80°C). Plasma TFNα and IL-6 were measured by ELISA technique (Human TNF-alpha Quantikine HS ELISA and Human IL-6 Quantikine HS ELISA kits respectively, R&D Systems Europe, Abington, UK). The inter-assay coefficients of variation were 12.74% for TNFα and 13.09% for IL-6, respectively. The intra- assay coefficients of variation were 19.04% and 11.09% for TNFα and IL-6, respectively. The lower detection limits for TNFα and IL-6 were 0.209 and 0.133 pgr/ml, respectively.

Control variables

Demographic variables (sex, age, years of formal education), BMI, presence of sleep disordered breathing, and diagnosis of late-life depression based on the Phase II clinical interview and scores on self-report psychiatric symptom scales served as covariates in the analyses. Additionally, prescription of any type of psychotropic medication, i.e., antipsychotic, antidepressants, and anxiolytics/hypnotics (including benzodiazepines), were included as covariates. However, specifically benzodiazepines, also served as covariates in separate models, due to their dual effect on sleep/sleepiness and cognition.

Statistical analyses

Diagnostic group differences (dementia, NCI) on the main objective index of sleep duration (24 h total sleep time [TST] averaged over the 3-day recorded period), daytime TST, nighttime sleep latency, and daytime sleep latency were assessed through ANCOVAs controlling for age, sex, BMI, sleep disordered breathing, depression, and use of psychotropic medications (evaluated at Bonferroni-adjusted alpha = 0.0125). Group differences on inflammation markers (IL-6 and TNFα) were also assessed through similar ANCOVA models (evaluated at alpha = 0.025). Furthermore, given the difference in antidepressant medication use between the two groups, we examined the effect of antidepressant medications on sleep/sleepiness after excluding participants from both groups who were on such medications.

Finally, associations between IL-6, TNFα, and sleep latency with age- and education-adjusted memory/IADL scores were assessed through multivariate, hierarchical regression models. Covariates associated with each dependent variable at p < 0.1 were entered in the first step (among sex, BMI, depression, sleep disordered breathing, and use of psychotropic medications), and IL-6, TNFα, or sleep latency was entered in the second step.

In view of literature suggesting non-linear association between sleep duration and cognitive function, hierarchical regression models examining the association between 24 h/daytime TST with age- and education-adjusted memory/IADL scores were computed following the steps described above, whereas the last step also included the quadratic effect of 24 h TST. Each of the aforementioned models was computed separately in the Dementia and NCI groups, and relevant regression coefficients were evaluated at Bonferroni-adjusted, family-wise alpha = 0.05/8 = 0.0063. Given the relatively small number of Dementia and NCI participants who registered naps during the recording period, the effect of daytime sleep on cognitive and daily function was assessed via one-way ANCOVAs adjusted for age, sex, BMI, depression, sleep apnea, and use of psychotropic medications comparing the two subgroups of patients with dementia (nappers versus non-nappers; evaluated at Bonferroni-adjusted alpha = 0.013).

RESULTS

Sample characteristics

The final sample included 131 participants with a diagnosis of dementia (N = 46) and 85 NCI subjects. The majority of dementia patients were diagnosed with probable AD (n = 43), and the remaining 3 patients with mixed dementia (both AD and vascular dementia features).

As shown in Table 1, the two groups were comparable on sex distribution, percentage of persons with benzodiazepine use, BMI, self-reported anxiety symptomatology, and frequency of late-life depression diagnosis (although there was a trend toward more severe self-reported depression symptoms among persons with dementia). As expected, dementia patients were significantly older than NCI. In addition, dementia patients had attained significantly fewer years of formal education than NCI participants. Furthermore, a higher percentage of dementia patients were using psychotropic medications compared to the NCI group. Average performance on the memory tests and functionality scale for each group are shown in Supplementary Table 1.

Table 1

Demographic and clinical information by diagnostic group

	Dementia (n = 46)	Non-cognitively impaired (n = 85)	p
Age (y)	80.3±5.6	73.0±7.4	<0.001
Education (y)	4.4±3.1	5.7±3.0	0.019
Sex (%)
Men	40%	37%	0.4
Women	60%	63%	0.4
CDR impairment class (%)
Unimpaired	5.3	85.4	<0.001
Questionable	78.9	14.6	<0.001
Moderate/Severe	15.8	–	<0.001
MMSE²	19.9±3.5	26.6±3.0	<0.001
GDS¹	4.3±3.8	3.0±3.2	0.07
HADS¹	3.6±3.3	2.7±3.6	0.2
Depression (%)	19.6	16.5	0.4
Psychotropic medication (%)	60.9	23.5	<0.001
Benzodiazepines (%)	15.2	11.8	0.3
Antidepressants (%)	26.0	12.9	0.09
BMI¹	30.0±4.7	30.2±5.2	0.8

CDR, Clinical Dementia Rating (Morris, 1993); MMSE, Mini Mental State Examination (Folstein, Folstein, & McHugh, 1975; [24]); GDS, Geriatric Depression Scale (Fountoulakis et al, 1999); HADS, Hospital Anxiety and Depression Scale (Mystakidou et al, 2004; Anxiety subscale); BMI, body mass index; ¹Age-adjusted values; ²Age- and education-adjusted values. p Values are for t-tests for continuous variables and chi-square tests for proportions.

Sleep measures

Subjective sleep data are shown in Table 2. Self-reported EDS was more frequent among patients with dementia versus participants in the NCI group (p = 0.05), although frequency of sleep disordered breathing symptoms did not differ between the two groups.

Table 2

Subjective and objective, sleep characteristics and inflammatory markers by diagnostic group

	Dementia (n = 46)	Non-cognitively impaired (n = 85)	p
nTST (min)	446.0±92.5	418.6±77.0	0.12
nTIB (min)	564.9±87.7	516.7±88.6	0.02
nSL (min)	15.0±14.4	11.5±6.7	0.1
dTST (min)	76.7±30.9	60.9±30.7	0.2
dSL (min)	13.8±9.9	10.5±9.0	0.3
24 h TST (min)	491.2±107.1	444.6±88.5	0.027
24 h TIB (min)	637.2±128.4	561.8±113.5	0.005
Self-Reported EDS (%)	8.9	1.2	0.05
Sleep Disordered Breathing symptoms (%)	26.1	18.8	0.3
IL-6 (pgr/ml)	1.5±1.1	1.4±0.9	0.5
TNFα (pgr/ml)	1.3±0.6	1.1±0.5	0.2

Data are mean±SD adjusted for age, sex, BMI, depression, use of psychotropic medications, except self-reported data (% of participants reporting sleep complaints); Naps were recorded in 25 dementia, and 39 NCI participants. p values are for ANCOVAs for continuous variables and Fisher’s exact tests for proportions. nTST, night Total Sleep Time; nSL, night sleep Latency; dTST, day Total Sleep Time; dSL, day sleep Latency; 24 h TST, 24 hour Total Sleep Time; 24 h TIB, 24 hour Total Time in Bed; EDS, excessive daytime sleepiness; IL-6, interleukin 6; TNFα, tumor necrosis factor α.

Analysis of objective sleep parameters showed that in patients with dementia sleep and time in bed overall was marginally longer and more disturbed, compared to the NCI group (Table 2), as indicated by main effects of Diagnostic Group (NCI, dementia) approaching significance on 24 h TST, F(1,123) = 5.026, p = 0.027, nTIB, F(1,123) = 5.089, p = 0.026, and 24 h TIB, F(1,123) = 8.245, p = 0.005. In terms of daytime sleep duration, dementia patients compared to NCI participants slept longer during the day by about 15 minutes, although this difference did not reach significance (p > 0.3). We also compared dTST between the two diagnostic groups using nTST as an additional covariate (including only nappers). The difference between the dementia (mean dTST = 76.3, SE = 7.3 min) and NCI groups (mean dTST = 62.0, SE = 5.2 min) failed to reach significance (p > 0.1).

Moreover, based on the higher use of antidepressants in dementia patients compared to NCI, we examined the hypothesis that higher daytime sleep/sleepiness in dementia patients may be related to the medication use. Therefore, we repeated the analysis after excluding participants on antidepressants from both groups. As shown in Supplementary Table 4, the results in terms of sleep remained the same, after adjusting for age and use of psychotropic medications.

Finally, because of the age differences observed between the dementia and the NCI group, that may have a significant effect on sleep and inflammation patterns we additionally performed an analysis comparing objective and subjective sleep as well as inflammation markers between dementia patients and an age-matched subgroup of NCI participants and results remained the same (Supplementary Table 3). The two groups did not differ on sex (p = 0.2), education (p = 0.3), BMI (p = 0.8), and depression (p = 0.9), although dementia patients were significantly more likely to take psychotropic medications (58.5 versus 19.5%, p = 0.001).

Associations between daytime napping and neurocognitive performance among patients with dementia

Regression coefficients between inflammation indices and memory/IADL scores did not reach significance in either group (p > 0.2 in all cases). Standardized regression coefficients (adjusted for appropriate covariates) are listed in Table 3. Significant associations between objective sleep indices and memory/IADL scores were found only among patients with dementia. Specifically, the association between 24 h TIB and both autobiographic memory and IADL followed a negative linear pattern (with the quadratic effect not approaching significance). However, the association between 24 h TST with IADL and two episodic memory indices (AVLT Retention, autobiographic memory) was curvilinear as indicated by significant, negative quadratic effects. These effects are illustrated in Fig. 2.

Table 3

Standardized regression coefficients for age- and education-adjusted memory/IADL scores regressed over inflammatory indices, objective sleep and per diagnostic group

	IL-6	TNF-a	24 h TST¹	24 h TST²	24 h TIB¹	24 h TIB²
Dementia
Digits Reverse	–0.103 (0.3)	–0.027 (0.8)	–0.313 (0.03)	–0.035 (0.8)	–0.239 (0.084)	–0.092 (0.5)
AVLT Retention	–0.187 (0.1)	–0.092 (0.4)	–0.010 (0.9)	–0.355 (0.003)	–0.229 (0.1)	–0.071 (0.7)
Autobiographic memory	–0.158 (0.2)	–0.095 (0.4)	–0.371 (0.002)	–0.343 (0.003)	–0.408 (0.001)	–0.060 (0.6)
IADL scale	–0.102 (0.3)	–0.043 (0.6)	–0.309 (0.013)	–0.287 (0.023)	–0.537 (0.0001)	–0.024 (0.8)
Non-cognitively impaired
Digits Reverse	–0.126 (0.4)	–0.171 (0.3)	–0.021 (0.8)	0.072 (0.5)	–0.032 (0.8)	–0.052 (0.6)
AVLT Retention	–0.211 (0.2)	–0.060 (0.4)	–0.183 (0.09)	0.037 (0.7)	–0.180 (0.1)	0.164 (0.1)
Autobiographic memory	–0.066 (0.6)	–0.060 (0.7)	–0.058 (0.5)	0.014 (0.8)	–0.158 (0.1)	0.123 (0.2)
IADL scale	–0.039 (0.7)	–0.058 (0.7)	–0.152 (0.1)	–0.049 (0.6)	–0.189 (0.08)	–0.035 (0.7)

p-values in parentheses. ¹Group-mean centered values. ²24 h TST/TIB centered, squared. Values have been adjusted for sex, BMI, depression, sleep Disordered Breathing, or use of psychotropic medications. AVLT, Auditory Verbal Learning Test; IADL, Greek Independent Activities of Daily Living scale (informant); 24 h TST, 24-hour Total Sleep Time; 24 h TIB, 24-hour Total Time in Bed; IL-6, interleukin 6; TNFα, tumor necrosis factor α.

Fig.2

Scatter plots of age- and education-adjusted AVLT Retention scores regressed over 24 h TST values for patients with dementia (left-hand panel) and NCI participants (right-hand panel). R² = 0.166 (p = 0.011) for the linear plus quadratic effect among dementia patients (adjusted for age, sex, BMI, depression, sleep apnea and use of psychotropic medications). Corresponding R² = 0.041 (p = 0.07) for the linear-only effect among NCI participants (p > 0.7 for the quadratic effect). Dotted lines represent 95% confidence intervals of the estimate. AVLT, Auditory Verbal Learning Test; 24 h TST, 24 hour Total Sleep Time; NCI, non-cognitively impaired.

We also compared patients with dementia who registered at least one nap during the three-day actigraphy recording (n = 25) and those who did not register daytime sleep during this period (n = 21). One-way ANCOVAs adjusted for age, sex, BMI, depression, sleep disordered breathing, and use of psychotropic medications comparing the two subgroups of patients with dementia reached significance (evaluated at Bonferroni-adjusted p < 0.013). Specifically, the former subgroup showed significantly lower performance on all memory tests as compared to the latter group (see Table 4). IL-6 or TNF-a levels did not vary significantly between nappers and non-nappers (p > 0.7).

Table 4

Memory test scores as a function of daytime nap habits in dementia patients

	Nappers (n = 25)	Non-nappers (n = 21)	p
Digits Reverse (0–24)	3.33 (2.89)	5.28 (2.04)	0.007
AVLT Retention (0–1.0)	0.033 (0.15)	0.16 (0.23)	0.010
Autobiographic memory (0–16)	9.47 (3.21)	11.72 (2.45)	0.002
IADL scale (0–100)	52.13 (30.20)	70.46 (22.80)	0.012

Means were adjusted for age, education, sex, BMI, depression diagnosis, sleep Disordered Breathing, and use of psychotropic medications. Range of possible score on each test are given in parentheses. AVLT, Auditory Verbal Learning Test; IADL, Greek Independent Activities of Daily Living scale (informant).

Among NCI participants, nappers (n = 46) and non-nappers (n = 39) showed comparable levels of pro-inflammatory cytokines (p > 0.7).

Associations between inflammation with objective nighttime and daytime napping

No significant differences were found between patients with dementia and NCI participants on either IL-6 or TNFα levels (Table 2). In the sub-sample of participants not on anti-depressant medication, the results in terms of inflammation markers remained the same, after adjusting for age and use of psychotropic medication (Supplementary Table 4).

Among patients in the dementia group IL-6 correlated positively with 24 h TST (partial r = 0.322, p = 0.01) and dTST (partial r = 0.500, p = 0.01) and negatively with nSL (r = –0.399, p = 0.01), indicating that increased levels of IL-6 were associated with increased objective sleep duration, especially during daytime, and sleepiness controlling for age, sex, BMI, sleep disordered breathing, depression, and use of psychotropic medications and/or benzodiazepines. Notably, the association between nTST and IL-6 did not reach significance (r = 0.11, p = 0.4).

Associations between TNFα and objective sleep were negligible in either group (r < 0.1) as were all correlations between inflammation markers and any of the objective sleep/sleepiness variables measures in the NCI group.

Finally, within the dementia patients, IL-6 or TNFα levels did not vary significantly between nappers and non-nappers (p > 0.7).

DISCUSSION

The main finding of this study is that in patients with mild to moderate dementia, objective daytime napping is associated with worse cognitive performance. Moreover, among those patients who napped, daytime sleep duration was associated with higher peripheral levels of the pro-inflammatory cytokine IL-6.

Objective daytime napping is associated with cognitive performance in patients with dementia

Very few, small studies in the past have examined objective daytime sleep/sleepiness in dementia patients and the findings were inconsistent. Two studies using polysomnogram (PSG) recording found that patients with dementia compared to controls were sleepier during the day as indicated by shorter sleep latency in the Multiple Sleep Latency Test (MSLT) or higher rates of napping during the day [9, 37], while another small study failed to find increased sleepiness in this group of patients [7]. A recent large, longitudinal, community-based study on elderly men without cognitive impairment showed that objectively long napping had higher risk of cognitive impairment at follow-up [38]. Our study showed that objective daytime sleep is longer in mild to moderate dementia patients compared to non-cognitively impaired elderly, and is associated with cognitive performance. Specifically, in this study we found that patients with dementia slept 15 minutes longer during the day compared to NCI controls. It should be noted that increased napping among patients with dementia was not associated with sleep deprivation, since previous night TST was increased. Our findings are consistent with the study by Leng et al. reporting that the association between napping and cognitive impairment was more pronounced among those with higher nighttime sleep efficiency and sleep duration. Our study adds to the above finding, by linking higher inflammatory markers to prolonged daytime napping [38]. Interestingly, a recent study reported damage to wake-promoting nuclei in patients with AD [39]. Furthermore, few studies in the past have examined associations between objective sleep/sleepiness in patients with dementia. These studies reported that short sleep latency in MSLT is adversely associated with several cognitive measures [9], and that nap time/TST (% of sleep time) increased with disease severity [37]. These are consistent with our findings suggesting that objective daytime napping in patients with dementia is associated with worse cognitive performance. Specifically, in this study we found that memory scores as well as assessment of daily life activities were more impaired among patients with dementia who napped during the day compared to the non-nappers. It is plausible that increased sleepiness during the day among patients with dementia may be related to impaired arousal mechanisms evident in the early stages of the disease [9].

Objective daytime napping is associated with increased IL-6 levels in patients with dementia

Only one previous study in patients with AD has examined the association between daytime sleep/sleepiness and inflammation. This study used only subjective measures of sleepiness, i.e., the Epworth Sleepiness Scale (ESS) and reported an association between TNFα and ESS. [22] Our study is to our knowledge the first to report that in patients with dementia, objective daytime napping is proportional to IL-6 levels. Given that inflammation is a prognostic factor of increased morbidity and mortality, daytime napping, measured with a convenient, inexpensive, ecologically valid method, such as actigraphy, can be a useful parameter in terms of predicting severity of dementia.

At a molecular level, AD is closely linked to basal forebrain cholinergic dysfunction which plays a key role within the central arousal system, regulating the sleep-wake cycle [40, 41]. Thus, it has been hypothesized that objective daytime sleep may be due to the weakness of the cholinergic system evident even in the early stages of AD [9].

Amyloid-β (Aβ) accumulation in cholinergic basal forebrain structures (a pathological hallmark of AD) has been linked to disturbed sleep patterns in patients with dementia (or individuals at risk of developing dementia) [42]. Conversely, there is evidence that normal sleep can contribute to brain Aβ clearance [43], whereas deranged sleep enhances Aβ deposition [44]. Also, while Aβ is increasingly deposited in brain, its levels in the cerebrospinal fluid (CSF) gradually drop during transition from normal cognition to mild cognitive impairment and then to AD [45]. In this respect, it has been shown that middle-aged individuals with low CSF Aβ levels have deranged sleep, characterized among others by more daytime naps [46]. Taken together, all these results suggest that sleep impairment and AD may share common pathogenetic mechanisms [47].

The possible role of inflammation in the pathophysiology of dementia has been suggested by studies that have shown that: 1) IL-6 and TNFα are elevated in patients with AD even in early stages of the disease [15 , 48–55], 2) soluble IL-6 receptors are decreased among patients with AD [56], and 3) mutations in the IL-6 genes were linked to increased risk for dementia [57, 58]. Also, it has been reported that in patients with cognitive impairment, levels of TNFα receptors in peripheral blood or CSF are increased compared to cognitively non-impaired controls [49 , 59]. In our study, plasma concentrations of pro-inflammatory cytokines IL-6 and TNFα were not significantly elevated among patients with dementia compared to cognitively non-impaired controls. Lack of significant increase in cytokines’ levels in our sample may be explained by the fact that they were measured in the morning, when the plasma levels of these cytokines are lowest (nadir) [60, 61]. Other explanations may be 1) the small sample size, 2) the washout effect based on the timing of collection relative to people’s internal clocks, and 3) the fact that the effect is present predominantly in people with concomitant sleep disturbances and cognitive impairment rather than those with only one or none of the two.

Some but not all studies report that IL-6 and TNFα are negatively associated with cognitive performance [49 , 64]. In our study, we failed to find significant associations between inflammatory markers and cognitive performance. Such inconsistencies are expected given the relatively low effect size of the observed correlations. Future studies in larger samples are needed to clarify associations between inflammatory markers and neuropsychological test scores that take into account additional person-specific variables such as the APOE gene ɛ4 genotype. The latter, in addition to affecting objective sleep quality [65], is known to be associated with increased rates of Aβ deposition in the brain which in turn directly impact cholinergic function [66, 67].

In our study, we also found that 24 h sleep duration among patients with dementia is marginally increased compared to cognitively non-impaired elderly. This is consistent with our findings in a larger group from the same cohort [6] and several previous studies reporting prolonged sleep among AD patients [5 , 68, 69].

Studies in the past have reported an inverse U-curve association between subjective sleep duration and cognition in elderly [70]. Some but not all studies using objectively measured sleep duration have reported associations between long sleep and cognitive function in community-dwelling elderly [70], while only one study found that short sleep duration was weakly associated with lower global cognitive performance [71]. In this study associations were driven by elderly with dementia. The authors suggested that shorter sleep reflected disturbed sleep with increased wake after sleep onset (WASO) and lower sleep efficiency, rather than quantity per se. Our study showed that cognitive impairment is associated with both extremes of sleep duration. A possible explanation is that sleep loss may lead to cognitive impairment, whereas, prolonged sleep may be an indicator of worse cognitive performance among patients with dementia.

We speculate that in dementia sleep and cognition may share common underlying neuropathological basis. In our study, twice as many patients with dementia used antidepressants compared to NCI participants. This is consistent with studies reporting that co-morbid depression is the second most frequent neuropsychiatric symptom among patients with AD [72]. It is also well-known that there is an association between depression and neuroinflammation [73]. It is possible that the neuroinflammation reported in the demented participants of our study can be the common underlying pathway leading to cognitive impairment and higher frequency of depression in these subjects.

Clinical implications

Our findings have several clinical implications. Previous studies have reported that hippocampal atrophy measured with magnetic resonance imaging, altered cerebral metabolism measured with fluorodeoxyglucose positron emission tomography, and CSF levels of tau and p-tau, are biomarkers related to severity and prognosis of patients with dementia [74]. We now provide data that objective daytime napping, based on a convenient, inexpensive, ecologically valid method, i.e., actigraphy, is associated with adverse prognosis and severity in patients with mild to moderate dementia. No cure is currently available for dementia, and interventions focusing in delaying the progress of the disease in rather early stages may be useful. For example, preventive strategies targeting on modifiable factors may help curb the deterioration rate of dementia in patients with objective daytime napping. Furthermore, given the association between inflammation and cognition, it might be useful to explore treatments targeting inflammatory pathways. Finally, given the association of daytime sleep with cognition and disease severity, treatment with sedative psychotropic medications should be administered with caution in persons suffering from dementia [75].

Strengths and limitations

This study has notable strengths. These include the well-characterized sample based on a detailed neuropsychiatric and neuropsychological evaluation of patients with dementia, in which sleep was evaluated objectively. Furthermore, the sample of this study is a naturalistic one, including all community-dwelling dementia patients, without excluding various co-morbidities that may end up in a selected super “healthy” group of patients with dementia.

Our study also has certain limitations. First, the sample size was relatively small, and included three patients with mixed (vascular and AD dementia features co-existing). Of note, our findings did not change when we excluded these three patients. Second, in our sample a higher percentage of dementia patients used psychotropic medication, i.e., antipsychotic, antidepressant, compared to the NCI group. However, when we statistically corrected for the possible effect of psychotropics, including benzodiazepines and antidepressants, on cognition and daytime sleepiness the results remained the same. Third, given that the diagnosis of dementia was based on a thorough neuropsychiatric evaluation and neuropsychological assessment, the results of this study cannot be extrapolated to a clinical dementia population. Fourth, in our sample only 3 out of 46 were diagnosed with mixed dementia, while none was diagnosed with vascular or other neurodegenerative disease. This prevalence-wise is highly unlikely and may reflect diminished ability of clinicians to differentiate between dementias, given also that neuroimaging was not available for many of the patients examined. Fifth, sleep was recorded with actigraphy and not PSG, which is the gold-standard for objective sleep assessment, including breathing abnormalities. Furthermore, actigraphy was conducted for only 3 24 h periods, while in other studies 1 to 2 weeks of actigraphy recordings were used. Finally, the cross-sectional nature of the study does not permit conclusions regarding causality between sleep and inflammation.

Conclusion

In conclusion, the current results indicate that objective daytime napping is associated with worse cognitive performance and inflammation, as measured by IL-6 levels, among patients with dementia. We propose that daytime sleep in demented patients may be an indicator of disease severity. Future studies focusing on preventing or reversing inflammation at early stages among patients at risk for dementia, may be useful in improving or delaying cognitive decline.

Footnotes

ACKNOWLEDGMENTS

This study was supported by a grant from the National Strategic Reference Framework (ESPA) 2007-2013, Program: THALES, University of Crete, title: “A multi-disciplinary network for the study of Alzheimer’s Disease” (Grant no MIS 377299). Content of the manuscript is solely the responsibility of the authors.

We thank study coordinator Cynthia Manasaki for her continuing support.

Authors’ disclosures available online ().

The supplementary material is available in the electronic version of this article: .

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