Seasonal Fatigue

Energy and mood dipping as daylight shortens — the milder end of a spectrum that runs to seasonal affective disorder. It reflects circadian rhythm disruption from reduced light exposure, often compounded by vitamin D decline, and it responds best to light itself.

Evidence synthesis: Seasonal affective patterns are driven by light deprivation disrupting circadian rhythms and serotonin regulation — the latitude gradient in prevalence is well documented. Bright light therapy (10,000 lux for ~30 minutes each morning) has meta-analytic support with effect sizes comparable to antidepressants for SAD, and dawn-simulation devices have trial evidence too. Vitamin D levels fall through winter, deficiency correlates with low mood, but supplementation trials for seasonal mood have produced mixed results — reasonable to correct deficiency, not a proven mood treatment on its own. Behavioural levers with evidence: morning outdoor light exposure even on overcast days, consistent wake times, and aerobic exercise (which has its own antidepressant effect size in trials). CBT tailored to SAD shows durable benefit in randomised trials, with lower relapse than light therapy alone in follow-up.