No Evidence for Seasonal Variations in Fatigue, Sleepiness and Insomnia Symptoms: Spring Fatigue Is a Cultural Phenomenon Rather Than a Seasonal Syndrome.

Although not as prominent as in other animals, humans also experience seasonal variations in sleep duration and circadian processes. These variations are likely primarily driven by changes in photoperiod length. Anecdotally, many people report experiencing fatigue and low energy levels, particularly during spring in Germany, Switzerland and Austria. This phenomenon is commonly referred to as 'spring fatigue'. However, scientific evidence for such a seasonal syndrome is missing. We thus investigated temporal variations in fatigue, daytime sleepiness, insomnia symptoms and sleep quality through an online survey including nine assessments of the same individuals over 1 year. We hypothesised that fatigue and daytime sleepiness would be higher during shorter photoperiods. We further expected lower sleep quality and more severe insomnia symptoms under shorter photoperiods. Additionally, we explored variations with photoperiod change, across months and seasons. Hypotheses were tested using Bayesian linear mixed-effects models. The study and analyses were pre-registered. Between April 2024 and September 2025, 418 adults (80% women) completed at least two assessments. Nearly half of the participants (47%) reported experiencing spring fatigue. Repeated assessments across 1 year showed no evidence for seasonal or monthly variations in fatigue, sleepiness, insomnia symptoms or sleep quality. Fatigue during day-to-day activities decreased with longer photoperiods but was independent of photoperiod change. Overall, the results provide evidence against spring fatigue as a genuine seasonal phenomenon. The discrepancy between high self-reports of the phenomenon and stable longitudinal patterns suggests that spring fatigue may reflect cultural labelling and result from cognitive-perceptual biases, rather than being a genuine seasonal syndrome.