Overview
- Large population analyses, including UK Biobank sleep data, show people sleep longer yet less restfully in winter, reinforcing a seasonal shift in human biology.
- Researchers have documented seasonal changes in the expression of thousands of genes in blood and fat, aligning with reports of mood and energy fluctuations.
- Clinicians recommend starting bright light therapy early in the season, typically 10,000 lux for 30 to 60 minutes in the morning, to realign circadian timing.
- Cognitive behavioral therapy, regular exercise, consistent sleep routines, and antidepressants when indicated remain core treatments alongside light therapy.
- Risk is higher in northern latitudes, among women, and in people with bipolar disorder, and prevalence ranges from roughly 2–6% for SAD to a larger share with milder winter blues.