Natural summer and winter sunlight exposure patterns in seasonal affective disorder.

Twelve subjects with winter depression who lived in the Chicago area recorded their times of going outside during daylight hours for one week during the winter and one week during the summer. These records produced estimates of the duration of daily sunlight exposure and of perceived dawn, dusk and skeleton photoperiod. There was more than twice as much sunlight exposure in summer compared to winter (3.0 vs. 1.2 h/day). The perceived skeleton photoperiod was 4-5 h longer in summer than winter, with a later perceived dusk contributing more to the lengthening than an earlier perceived dawn. The duration of sunlight exposure and perceived skeleton photoperiod in both seasons was much less than what was possible given the available daylight. These results are discussed with reference to the modern urban life style, bright light treatment of winter seasonal affective disorder, and factors which affect the perceived intensity of sunlight.