Maintenance of a circadian phase adjustment of the human melatonin rhythm following artificial long days

In winter, a 5-day exposure of 4 human subjects to a skeleton photoperiod, with 3 h of bright light in the evening and again in the morning, phase advanced the morning serum melatonin offset by 1–3 h as compared with the original winter melatonin rhythm pattern. The phase advance persisted for 3 days even after the bright light withdrawal. The data indicate that the long skeleton photoperiod had a prevailing phase-advancing effect on the melatonin rhythm and its underlying pacemaker. The maintenance of the phase advance might be due to the fixed sleep-wake schedule.