Background Mood 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.

Methods We 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 ≥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).

Results Depressive (β=−0.67, p=0.023) and negative symptoms (β=−0.17, p=0.041) peaked in winter, whereas manic symptoms peaked in autumn (β=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 β=−0.06, p=0.006) and negative symptoms (indirect β=−0.05, p<0.001). Higher activity levels partially mediated season’s effect on depressive symptoms (indirect β=−0.010, p=0.032). Extended sleep with nocturnal activity mediated season’s effect on negative symptoms (indirect β=−0.02, p=0.001). No mediators emerged for manic symptoms.

Conclusions Light 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.

The authors have declared no competing interest.

This work is supported by a National Health and Medical Research Council (NHMRC) grant (2019206). This work was further supported by NHMRC EL1 Investigator Grants (Grant No. GNT2008196 [to JJC]) and an NHMRC L3 Investigator Grant (Grant No. GNT2016346 [to IBH]).

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Ethics approval was obtained from the University of Sydney Human Research Ethics Committee (2012/1631) for the YMH study and the Sydney Local Health District Human Research Ethics Committee (2020/ETH01272) for the Neurobiology study. All participants provided written informed consent; parental/guardian consent was also obtained for participants under 16 years.

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