Sixty-eight patients with seasonal affective disorder participated in a 10 000-lux light treatment study in which two questions were addressed: do response rates differ when the light is applied at different times of the day and does short-term rank ordering of morning and evening light influence response rates?

Three groups of patients received a 4-day light treatment: (I) in the morning (8.00–8.30 a.m., n = 14), (II) in the afternoon (1.00–1.30 p.m., n = 15) or (III) in the evening (8.00–8.30 p.m., n = 12). Two additional groups of patients received two days of morning light treatment followed by two days of evening light (IV, n = 13) or vice versa (V, n = 14).

Response rates for groups I, II and III were 69, 57 and 80% respectively, with no significant differences between them. Response rates for groups IV and V were 67 and 50% respectively; this difference was not significant and these percentages did not differ significantly from those of groups I and III.

The results indicate that the timing of light treatment is not critical and that short-term rank ordering of morning and evening light does not influence therapeutic outcome.