Bright artificial light has been found effective in reducing winter depressive symptoms of Seasonal Affective Disorder, although conclusions about the true magnitude of treatment effect and importance of time of day of light exposure have been limited by methodologic problems. Individual subjects' data from 14 research centers studying 332 patients over 5 years were analyzed with a pooled clustering technique. Overall, 2500-lux intensity light exposure for at least 2 hours daily for 1 week resulted in significantly more remissions--Hamilton Depression Rating Scale (HAM-D) score reduction of 50% or more to a level under 8--when administered in the early morning (53%) than in the evening (38%) or at midday (32%). All three times were significantly more effective than dim light controls (11%). Dual daily exposures (morning-plus-evening light) provided no benefit over morning light alone. In morning-evening crossovers, remission rates were 62% under morning light alone, compared with 28% under evening light alone, with a differential morning-evening response present in 59% of morning responders compared with 10% of evening responders (p less than 0.001). Remission rates with morning light were highest given low severity at baseline (HAM-D score of 10-16: 67% remission), as compared with moderate-to-severe cases (HAM-D score above 16: approximately 40% remission) where no morning-evening differences were found. Firmer conclusions await treatment studies with larger sample sizes and full assessment of atypical vegetative symptoms seen in winter depression but underrepresented in the Hamilton scale. Longer treatment course and greater light intensity may help clarify clinical response despite the impossibility of achieving a conventional blind placebo control.