Seasonal Variation in Mood and Associated Patterns of Sleep, Activity, Circadian Rhythms and Light

The mechanisms underlying seasonal mood variation are incompletely characterised. We tested whether wearable-measured sleep, activity, circadian and light exposure patterns are associated with seasonal mood variation in young Australians with emerging mood disorders. We analysed 733 observation periods from 422 participants (mean age 24.3 ± 5.5 years; 63% female) attending early-intervention clinics. Each observation included a clinical assessment paired with ≥5 valid days of 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 (JIVE). Light exposure features were reduced using principal component analysis (PCA) applied to light regularity, timing and exposure interval across 7 illuminance thresholds. Linear mixed-effects models tested seasonal effects on clinical outcomes; indirect associations for 12 candidate JIVE/PCA variables were estimated by cluster bootstrap. Depressive symptoms (β = -0.67, P = .023) and negative symptoms (β = -0.17, P = .041) peaked in winter, whereas manic symptoms peaked in autumn (β = 0.24, P = .019). Winter increases in negative symptoms were associated with an 'extended sleep with phase-delayed activity' profile (indirect β = -0.015, P < .002) and a 'constrained pattern of moderate-to-bright indoor light exposure' profile (indirect β = -0.038, P = .002), reflecting high day-to-day consistency with a narrow daily exposure window. The light association persisted after adjustment for time spent above the light threshold, which itself was not associated with symptoms. Winter increases in depressive symptoms were associated with lower overall physical activity levels (indirect β = -0.009, P = .040). No indirect associations were identified for manic symptoms. In this transdiagnostic youth sample, seasonal variation in mood was modest but was accompanied by differences in how personal light exposure was arranged across the day, not only in how much light was received and in daily activity patterns. These cross-sectional associations identify light patterning and activity rhythms as candidate modifiable targets for prospective and interventional research.