Low vitamin D? Supplement. It works with light therapy.

Vitamin D supplementation is worth it for late-winter SAD if your serum levels are actually low, and it likely enhances the effect of light therapy, though not everyone needs both.

Seasonal affective disorder in late winter is partly about light deprivation. Light therapy tackles that by hacking your circadian rhythm through your eyes. But the same lack of sun also means your skin isn’t making vitamin D. If a blood test shows you’re deficient — and many people in northern latitudes are by February — then supplementing can lift mood by correcting that separate physiological deficit.

The two treatments target different mechanisms. Light therapy works through melanopsin and circadian entrainment. Vitamin D acts on brain receptors that influence serotonin and dopamine. They don’t compete; they complement. Some studies have found that combining them reduces depressive symptoms more than either alone.

The key is knowing your levels. If you’re not deficient, extra vitamin D won’t