5-Methoxypsoralen increases evening sleepiness in humans: possible involvement of the melatonin secretion.

The acute and short-term effect of 5-methoxypsoralen (5-MOP) on daytime sleepiness was investigated in 12 healthy subjects according to a double-blind cross-over design. 5-MOP (40 mg/day) or placebo was orally administered (one week each) at 16.00 h. 5-MOP significantly increases daytime sleepiness 4 to 5 h after administration without affecting the subsequent sleep efficiency and duration. We found a positive correlation between sleepiness and melatonin levels. Our findings suggest that the melatonin secretion may play a role in the effect of 5-MOP on sleepiness in humans.