Seasonal changes in daylength (photoperiod) modify the duration of nocturnal melatonin (MT) secretion in many vertebrates. In some cases the changes in MT act as chemical signals that trigger photoperiodic induction of breeding and other seasonal phenomena. It is unclear whether, and to what extent changes in daylength modify the duration of human MT secretion. To address this question, I investigated whether the duration of human MT secretion could be altered by artificial photoperiods. I exposed eight healthy volunteers to a conventional "summer" photoperiod of 16 h light and 8 h darkness for 1 week and to a "winter" photoperiod of 10 h light and 14 h darkness for 4 weeks. As occurs in animals, the duration of nocturnal MT secretion in human beings was longer after exposure to the short photoperiod (12.5 +/- 1.8 vs. 10.3 +/- 0.8 h, t = 3.778, P less than 0.01). The duration of the sleep-phase (recorded by electroencephalogram) was also longer (11.0 +/- 0.8 vs. 7.7 +/- 0.2 h, t = 11.754, P less than 0.001). Whether such changes would lead to significant seasonal changes in human physiology and behavior under natural lighting conditions may be worthy of further investigation.