Light therapy for depression: present status, problems, and perspectives.

Bright white light (WL) improves depressive symptomatology in seasonal affective disorders (SAD). Different dosage regimens are effective: photoperiod extension (3, 2, or 1 hr WL at dawn and dusk); morning only (2, 1, or 1/2 hr); midday only (4 or 2 hr); and evening only (5 or 2 hr). Late evening WL may be deleterious. The placebo effect of WL has not yet been adequately resolved. Only SAD patients and not major depressive disorders have responded to WL. The mechanism of action of WL is unknown: however melatonin does not appear to play a major role.