Seasonal Variations in the Severity of ADHD Symptoms in the Dutch General Population

Abstract

Objective: This is the first study to examine self-reported seasonal differences in the severity of ADHD symptoms in adults from the general population. Method: Data were analyzed from N = 5,303 respondents participating in the second wave of the Netherlands Mental Health Survey and Incidence Study–2, a population-based study on mental health. ADHD symptoms were assessed using the Adult ADHD Self-Report Scale Screener. As indicators of the severity of ADHD symptoms, the total ADHD symptom score and inattention and hyperactivity subscale scores were examined. Results: Compared with participants who were assessed in autumn, total ADHD and inattention subscale scores were significantly higher among participants who were assessed in spring or summer; the hyperactivity subscale score was significantly higher in spring. Conclusion: We found seasonal variations in the severity of ADHD symptoms, which was highest in those assessed in spring and summer. Researchers should be aware of this in the diagnostic process.

Keywords

seasons ADHD symptom severity population-based study

Introduction

ADHD is characterized by symptoms of hyperactivity, impulsivity, and/or inattention (American Psychiatric Association [APA], 2013). The estimated prevalence of adult ADHD is 3.4% (range = 1.2%-7.3%) in an international population-based study performed in three less developed and seven developed countries (Fayyad et al., 2007).

Although seasonal variations in psychiatric symptoms, including depressive and anxiety symptoms, are regularly investigated (e.g., Magnusson, Axelsson, Karlsson, & Oskarsson, 2000; Winthorst, Post, Meesters, Penninx, & Nolen, 2011), only two studies have investigated the influence of seasonality on the severity of ADHD symptoms. The first study assessed seasonal changes in the number of self-reported ADHD symptoms in a community sample of children and adolescents in the United States and Puerto Rico. Weak seasonal variations were observed in ADHD symptom count in the whole sample with the lowest ADHD symptom count in summer (Kovalenko et al., 2000). The second study explored the effects of winter and summer daytime lengths on severity of ADHD symptoms in a small clinical sample of Canadian children with ADHD and a small control group without ADHD (Langevin & Ramde, 2012). The difference between the level of ADHD symptoms in children with ADHD between winter and summer was greater than the difference in the control group. This study also indicated that seasonal variations were not specific for one ADHD symptom domain (inattention or hyperactivity). Furthermore, two population-based studies provided indications of seasonal effects on ADHD symptomatology, while two clinical studies did not. The first population-based study found that during the winter months, the search frequency for information on ADHD on the Internet was higher than during the summer months, both in the United States and in Australia (Ayers, Althouse, Allem, Rosenquist, & Ford, 2013). Second, a population-based study by Arns et al. showed a lower prevalence of ADHD among children and adults in areas with high solar intensity across U.S. states and also across nine non-U.S. countries (Arns, van der Heijden, Arnold, & Kenemans, 2013). The results of these two population-based studies indicate that the severity of ADHD symptoms might be lower in summertime as compared with wintertime. However, two clinical studies in U.S. children with ADHD have demonstrated that there are no seasonal variations in serotonin levels in the central nervous system, which are, like dopamine, linked to impulsive behavior (Schulz, Newcorn, Schmeidler, & Halperin, 2002; Swedo et al., 1989). Altogether, these prior studies yield conflicting results, and none of these studies has directly examined seasonal differences in the severity of ADHD symptomatology in adults from the general population.

The aim of the present study is to investigate seasonal variations in the self-reported severity of ADHD symptoms in the general population. We used data from the Netherlands Mental Health Survey and Incidence Study–2 (NEMESIS-2), an epidemiological study among adults from the general Dutch population. We also investigated whether seasonal variations are specific for one symptom domain (inattention or hyperactivity).

Material and Method

Participants

Data were used from the second wave of the NEMESIS-2 at which current ADHD symptoms were measured in the total sample. NEMESIS-2 is a longitudinal cohort study on the prevalence, incidence, course, and consequences of psychiatric disorders in the Dutch general population aged 18 to 64 years at baseline (November 2007-July 2009). NEMESIS-2 is based on a multistage, stratified, random sampling of households, with one respondent randomly selected from each household. Insufficient fluency in Dutch was an exclusion criterion. The response rate for the first wave was 65.1%. This sample was nationally representative, although younger participants were somewhat underrepresented. All first-wave respondents were approached for follow-up, 3 years after the first wave from November 2010 to June 2012. Of this group, 80.4% (N = 5,303) was reassessed. Psychopathology was not related to loss to follow-up in the second wave, after adjustment for demographics (de Graaf, van Dorsselaer, Tuithof, & ten Have, 2013). The research proposal was approved by a medical ethics committee and all respondents gave written informed consent. Sampling methods have been reported more extensively elsewhere (de Graaf, ten Have, & van Dorsselaer, 2010).

Measures

Seasons

We had information on the date of assessment for all 5,303 participants. The date was categorized into each of the four seasons. The calendar dates for each season were as follows: spring: March 21 to June 20; summer: June 21 to September 20; autumn: September 21 to December 20; and winter: December 21 to March 20.

ADHD symptoms

Severity of ADHD symptoms was measured using the Adult ADHD Self-Report Scale (ASRS) screener version 1.1 (Kessler et al., 2005). The validated ASRS screener (Kessler et al., 2007) consists of six items: four items indicating inattention symptoms and two items indicating hyperactivity symptoms. The frequency of a symptom over the past 6 months was rated on a 5-point response scale ranging from 0 (never) to 4 (very often). A total score (range = 0-24) was derived by summing the response scores for all six items. Individuals with a total score of ≥14 were considered to have an indication for a diagnosis of ADHD. Inattention and hyperactivity subscale scores were obtained by summing the response scores for the four inattention symptoms (range = 0-16) and the two hyperactivity symptoms (range = 0-8), respectively. The ASRS screener has been shown to have moderate sensitivity (64.9%), excellent specificity (94.0%), and excellent total classification accuracy (91.5%; Kessler et al., 2007). The ASRS screener has been shown to outperform the full version of the ASRS (with 18 items measuring the frequency of all 18 Diagnostic and Statistical Manual of Mental Disorders [4th ed.; DSM-IV; American Psychiatric Association, 1994] ADHD symptoms) in terms of psychometric properties and classification accuracy (Kessler et al., 2005). Because the ASRS screener asked to report on ADHD symptoms over the past 6 months, the reports on the ADHD symptoms always cover two seasons. However, the report of ADHD symptoms may also reflect one’s current symptoms due to a recency effect, that is, the report of symptoms that have occurred most recently, and thus reflect the most recent season (Kovalenko et al., 2000). In this study, severity of ADHD symptoms was determined by the total score, and by the subscale scores on each symptom domain (inattention and hyperactivity) separately.

Possible covariates and confounders

Sociodemographic data were collected, including age, gender, educational level, living with a partner, employment situation, and urbanicity. Use of ADHD medication (stimulants, atomoxetine, and modafinil) in the past 12 months was determined. The presence of any comorbid mood (major depression, dysthymia, bipolar disorder), anxiety (panic disorder, agoraphobia, social phobia, specific phobia, generalized anxiety disorder), and/or substance use disorder (alcohol or drug abuse and dependence) within the past 12 months was established using the Composite International Diagnostic Interview (CIDI) version 3.0 (Kessler & Ustun, 2004). The CIDI is a fully structured interview based on the DSM-IV (American Psychiatric Association, 1994), which has shown to be a reliable and valid instrument (Haro et al., 2006).

Statistical Analyses

Sample characteristics were compared between seasons using a chi-square test. Severity of ADHD symptoms was tested between two seasons using the independent samples t-test. Furthermore, to assess the effects of seasons on severity of ADHD symptoms, we performed linear regression analyses, adjusting for age and gender. Based on our t-test results, autumn was the reference category for these analyses. The total ADHD symptom score, the inattention subscale score, and the hyperactivity subscale score were the dependent variables in separate models. The data were weighted to generate a nationally representative sample of the adult population. The following sociodemographic characteristics were used to construct the weighting factor: age, educational level, employment situation, living with a partner, and ethnicity. For all weighted analyses, we obtained robust standard errors based on the first-order Taylor-series linearization method to obtain correct 95% confidence intervals and p values. Analyses were carried out using Stata version 12.1 (Stata Corporation, the United States), which enabled us to control for the complex sampling and recruitment procedure of the study. A p value of ≤ .05 (two-sided) was considered statistically significant.

Results

Sample Characteristics

Table 1 shows sample characteristics by season of assessment. Due to holidays, fewer participants were assessed in summer as compared with the other seasons. There were no seasonal differences in sociodemographic characteristics or in the presence of any comorbid mood, anxiety, and substance use disorders in the past 12 months. Seasonal differences were observed in the self-reported use of ADHD medication, but only 11 participants reported ADHD medication use.

Table 1.

Sample Characteristics (%) of 5,303 Participants From the Netherlands Mental Health Survey and Incidence Study–2 (Second Wave), by Season of Assessment.

	Season				χ²	df	p
	Spring (n = 1,272)	Summer (n = 857)	Autumn (n = 1,538)	Winter (n = 1,636)	χ²	df	p
Gender (female)	51.4	51.3	47.3	49.0	3	5.97	.404
Age (years)
18-24	9.3	14.5	12.0	13.3
25-34	22.7	19.0	20.4	17.4
35-44	24.5	22.0	23.9	26.0
45-54	23.2	23.3	22.2	24.6
55-64	20.2	21.3	21.5	18.7	12	35.78	.073
Educational level
Primary, basic vocational	6.0	6.8	6.0	9.1
Lower secondary	21.4	22.6	20.6	24.7
Higher secondary	43.6	40.4	43.7	38.9
Higher professional, university	28.9	30.3	29.8	27.3	9	29.64	.071
Not living with a partner	29.8	32.3	28.3	30.3	3	4.12	.470
No paid employment	25.6	22.8	24.5	25.4	3	2.68	.669
Urbanicity
Very urban	22.8	24.6	20.7	21.6
Urban	27.6	29.1	25.4	26.7
Somewhat urban	17.1	16.9	18.3	20.5
Somewhat rural	21.3	21.1	21.6	19.8
Rural	11.2	8.3	14.0	11.4	12	29.66	.590
Use of ADHD medication	0	0.02	0.27	0.25	3	5.21	.034
12-month comorbidity
Any mood disorder^a	6.8	6.4	4.7	5.4	3	6.93	.280
Any anxiety disorder^b	7.3	8.8	5.2	6.2	3	12.25	.102
Any substance use disorder^c	3.2	5.4	3.2	4.8	3	10.96	.223

Note. Bold p values are significant with α ≤ .05. df = degrees of freedom.

Included a diagnosis of major depression, dysthymia, and/or bipolar disorder.

Included a diagnosis of panic disorder, agoraphobia, social phobia, specific phobia, and/or generalized anxiety disorder.

Included a diagnosis of alcohol and/or drug abuse or dependence.

Seasonality in Severity of ADHD Symptoms

Based on the ASRS screener, the prevalence of an indication for a diagnosis of ADHD was 2.7% (results not tabulated). Figure 1A shows that the total ADHD symptom score was significantly higher among participants who were assessed in spring or summer as compared with participants who were assessed in autumn. Similar seasonal differences were observed with respect to the inattention subscale score (see Figure 1B), whereas the hyperactivity subscale score was only significantly higher among participants who were assessed in spring as compared with those assessed in autumn (see Figure 1C).

Figure 1.

(A) ADHD symptoms: total mean score per season. (B) Inattention symptoms: subscale mean score per season. (C) Hyperactivity symptoms: subscale mean score per season (N = 5,303).

Table 2 presents linear regression models in which we looked at the effect of seasons on severity of ADHD symptoms, adjusted for age and gender. We observed a positive effect on the total ADHD symptom score in spring as compared with autumn, and also in summer as compared with autumn. When specified into symptom domains, there was a positive effect on the inattention subscale score in spring and summer as compared with autumn, and a positive effect on the hyperactivity subscale score in spring as compared with autumn.

Table 2.

Linear Regressions With Total ADHD Symptom Score, Inattention Subscale Score, and Hyperactivity Subscale Score as Dependent Variables and Season of Assessment as Independent Variable, Adjusted for Age and Gender (N = 5,303).

	Total ADHD symptom score		Inattention subscale score		Hyperactivity subscale score
	B (SE)	p	B (SE)	p	B (SE)	p
Autumn (reference)
Winter	0.22 (0.17)	.177	0.09 (0.12)	.477	0.14 (0.09)	.108
Spring	0.58 (0.19)	.003	0.35 (0.14)	.014	0.22 (0.09)	.015
Summer	0.46 (0.18)	.011	0.32 (0.12)	.008	0.14 (0.10)	.167

Note. Bold p values are significant with α ≤.05.

Discussion

In this large population-based study, we examined seasonal variations in the self-reported severity of ADHD symptoms. We found that in participants who were assessed in spring and summer, the severity of ADHD symptoms was higher. The severity of the inattention symptom domain was also increased in spring and summer, and the severity of the hyperactivity symptom domain was increased in spring.

To our knowledge, this is the first study to examine self-reported seasonal differences in the severity of ADHD symptoms in adults from the general population. The first finding of a greater severity of ADHD symptoms in spring and summer contrasts to the findings of previous studies, which found a higher symptom severity in wintertime as compared with summertime (Kovalenko et al., 2000; Langevin & Ramde, 2012). Also, the second finding of a higher severity of inattention symptoms in spring and summer and of hyperactivity symptoms in spring is not in line with earlier findings by Langevin and Ramdé, who reported less ADHD symptoms in both symptom domains in summer as compared with winter in Canadian children with ADHD (Langevin & Ramde, 2012). These discrepant findings can be due to differences in study population and sample size. The present study included a large sample of the general adult population in the Netherlands, whereas the previous studies were based on a smaller community sample of 1,285 children and adolescents in the United States and Puerto Rico and on a small clinical sample of five Canadian children. In addition, the design of the study by Langevin and colleagues (Langevin & Ramde, 2012) was different, because they measured ADHD symptoms in the same sample of children both in winter and in summer.

How can we explain our finding of a greater severity of ADHD symptoms in spring and summer as compared with autumn? First, the increase in solar intensity during spring and summer days may have influenced one’s brain serotonin concentrations (Lambert, Reid, Kaye, Jennings, & Esler, 2002), which in turn may have resulted in physical restlessness, thereby mimicking ADHD symptoms (Goldberg, 1998). Second, because the ASRS asked to report on ADHD symptoms over the past 6 months, the reports on ADHD symptoms may cover two seasons. So, the increased severity of ADHD symptoms in spring and summer may reflect an increased severity in winter, spring, and summer. Given that peak values in serum brain-derived neurotrophic factor (BDNF) values have been observed in early autumn (Molendijk et al., 2012), and that low levels of BDNF, which is involved in the development of the dopamine system, may play a role in the etiopathogenesis of ADHD (Corominas-Roso et al., 2013), the lower severity of ADHD symptoms in autumn may be due to high BDNF during that season. Third, allergic rhinitis may have resulted in the greater severity of ADHD symptoms in spring and summer, because symptoms of allergic rhinitis may mimic ADHD symptoms (Chou et al., 2013). In the Netherlands, symptoms of allergic rhinitis exacerbate in spring and summer (de Bot et al., 2009). Finally, seasonal patterns in suicides (Coimbra et al., 2016) and manic episodes (Geoffroy, Bellivier, Scott, & Etain, 2014) have been repeatedly reported, showing also peaks in spring and in spring/summer, respectively. The underlying causes are not fully understood. However, some factors have been suggested as contributing to these seasonal fluctuations, including climatic factors (i.e., day length, temperature, humidity), viruses, psychiatric conditions, and socioeconomic factors (Lee, Tsai, & Lin, 2007; Woo, Okusaga, & Postolache, 2012). These factors may also be considered to play a role in the seasonality of ADHD symptoms. Clearly, further studies are required to elucidate the mechanisms responsible for the observed seasonal differences in ADHD symptoms.

Our study included a large sample size, and a validated instrument was used to measure ADHD symptom severity. However, four limitations of the study need consideration. First, we used self-reports of ADHD symptom severity instead of clinician’s ratings of ADHD symptom severity. Also, the ASRS screener does not assess the onset of ADHD symptoms in childhood, which is one of the diagnostic criteria of ADHD (APA, 2013). The observed prevalence of an indication for a diagnosis of ADHD is somewhat low in comparison with a previously reported adult prevalence estimate (Fayyad et al., 2007). Nevertheless, the ASRS showed good concordance with clinician diagnoses (Kessler et al., 2007). Second, ADHD symptoms may mimic symptoms of anxiety and depression, such as concentration problems and forgetfulness. Yet, a study (Milberger, Biederman, Faraone, Murphy, & Tsuang, 1995) showed that ADHD is not an artifact of symptoms shared with major depression, bipolar disorder, or generalized anxiety disorder and that these comorbid conditions themselves are not an artifact of overlapping ADHD symptoms. Third, and in line with the previous limitation, symptoms of ADHD and seasonal affective disorder (SAD) overlap (Amons, Kooij, Haffmans, Hoffman, & Hoencamp, 2006). Therefore, seasonal variations in ADHD symptoms may be due to the presence of SAD, although seasonal variations in ADHD symptoms may then be expected in those assessed in fall and winter, because ADHD is especially related to autumn/winter SAD (Amons et al., 2006; Levitan, Jain, & Katzman, 1999). Finally, because the ASRS measures ADHD symptoms over the past 6 months, the reports on ADHD symptoms may cover two seasons. Future studies on seasonal variations should use a questionnaire that covers only one season.

In conclusion, we found that there were seasonal variations in the self-reported severity of ADHD symptoms, which was highest in those assessed in spring and summer. The results suggest that researchers should be aware of seasonal variations in ADHD symptom severity in the diagnostic process of ADHD in the adult general population.

Footnotes

Declaration of Conflicting Interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: The Netherlands Mental Health Survey and Incidence Study-2 (NEMESIS-2) is conducted by the Netherlands Institute of Mental Heath and Addiction (Trimbos Institute) in Utrecht. Financial support has been received from the Ministry of Health, Welfare and Sport, with supplement support from the Netherlands Organization for Health Research and Development (ZonMw) and the Genetic Risk and Outcome of Psychosis (GROUP) investigators. The funding sources had no further role in the study design; in the collection, analysis and interpretation of data; in the writing of the report; or in the decision to submit the paper for publication.

Author Biographies

Suzan W. N. Vogel, MD, is PhD student at the PsyQ Expertise Center Adult ADHD in The Hague, The Netherlands.

Margreet L. ten Have, PhD, is senior researcher at the Netherlands Institute of Mental Health and Addiction in Utrecht and Principal Investigator of the NEMESIS-2 Study. Her expertise covers longitudinal research on adults’ mental health. She has (co-)authored over 100 international papers on prevalence, incidence and consequences of a variety of mental disorders among adults in the general population.

Denise Bijlenga, PhD, is psychologist and senior researcher at the PsyQ Expertise Center Adult ADHD in The Hague, The Netherlands. She earned her PhD in medical decision-making in the area of obstetrics at the Academical Medical Center of the University of Amsterdam. Her expertise covers social research methodology and statistics, and research project management.

Ron de Graaf, PhD, is senior researcher at the Netherlands Institute of Mental Health and Addiction in Utrecht and was Principal Investigator of the NEMESIS-2 study. His expertise covers longitudinal research on adults’ mental health. He (co-)authored over 300 international papers on prevalence, incidence and consequences of a variety of mental disorders among adults in the general population.

Aartjan T. F. Beekman, PhD, is psychiatrist and head of the department of Psychiatry at VU University Medical Center.

J. J. Sandra Kooij, PhD, is psychiatrist and head of Expertise Center Adult ADHD in The Hague, The Netherlands. In 2006 she received her PhD on clinical assessment and treatment studies in adult ADHD. Her current research areas include ADHD in older adults, general health, sleep, seasonal depressive symptoms, sexuality, and lifestyle in adult ADHD.

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