Seasonality and ADHD: Summer time is associated with less symptoms of inattention among children and adolescents with ADHD

Background: Although alertness and activity status are related to light intensity, the effects of seasonal light variability on patients with Attention Deficit Hyperactivity Disorder (ADHD) have been scarcely studied. Our objective was to investigate the effect of seasonality (summer time vs. winter time) on ADHD symptomatology and on the circadian pattern of motor activity in children and adolescents with ADHD and healthy controls. Methods: A total of 117 participants (56.4 % males; age [mean ± SD] 9.2 ± 2.8 years; 51 % patients with ADHD) were included in this cross-sectional study. For all participants we assessed ADHD symptomatology using the Conners Parents Rating Scale-Revised and the circadian pattern of motor activity using actigraphy. Data were collected when schools were in session. General linear models were used to test for differences in outcome variables based on seasonality and ADHD subtype. Results: The results revealed that seasonality had a significant effect among ADHD-I patients. Particularly, we observed that inattention symptoms were ~8.1 points lower (p = 0.002) in summer time, relative to winter time. In addition, we observed that among patients with ADHD-I, the intradaily variability was also significantly lower in summer (p = 0.024). Conclusion: The results from this preliminary study suggest that seasonal light variability could be a factor that modifies the behavior of children and adolescents with ADHD-I.