Seasonal variation in mood and the dynamics of sleep, activity, circadian rhythms, and light

BackgroundMood symptoms vary seasonally, yet the underlying mechanisms remain unclear. We tested whether wearable-derived sleep, activity, circadian, and light exposure patterns mediate seasonal effects on mood in youth with emerging mood disorders. MethodsWe analysed 733 observation periods from 422 Australian youth (mean age 24.3{+/-}5.5 years; 63% female) attending early-intervention mental health services. Each observation comprised a clinical assessment paired with [&ge;]5 valid days of GENEActiv wrist actigraphy. Season was modelled using sine-cosine functions of day-of-year. Sleep, activity, and circadian features were reduced using Joint and Individual Variation Explained, and light exposure features were reduced via principal components analysis. Linear mixed-effects models tested seasonal effects on depressive, psychiatric, manic, and functional outcomes. Mediation was examined using Sobel screening followed by cluster bootstrapping (1,000 iterations). ResultsDepressive ({beta}=-0.67, p=0.023) and negative symptoms ({beta}=-0.17, p=0.041) peaked in winter, whereas manic symptoms peaked in autumn ({beta}=0.24, p=0.018). Reduced day-to-day variability in moderate-to-bright ambient light exposure (fewer transitions to brighter environments) mediated winter increases in depressive (indirect {beta}=-0.06, p=0.006) and negative symptoms (indirect {beta}=-0.05, p<0.001). Higher activity levels partially mediated seasons effect on depressive symptoms (indirect {beta}=-0.010, p=0.032). Extended sleep with nocturnal activity mediated seasons effect on negative symptoms (indirect {beta}=-0.02, p=0.001). No mediators emerged for manic symptoms. ConclusionsLight exposure variability--reflecting constrained engagement with brighter environments during winter--emerged as the dominant mediator of seasonal mood worsening in Australian youth, with smaller contributions from sleep-activity-circadian patterns. These findings identify daily light variability as a promising, modifiable target for intervention.