Effects of melatonin administration on daytime sleep after simulated nightshift work

Disturbed sleep and on-the-job sleepiness are widespread problems among nig ht shift workers. The pineal hormone melatonin may prove to be a useful tre atment because it has both sleep-promoting and circadian phase-shifting eff ects. This study was designed to isolate melatonin's sleep-promoting effect s, and to determine whether melatonin could improve daytime sleep and thus improve night time alertness and performance during the night shift. The st udy utilized a placebo-controlled, double-blind, cross-over design. Subject s (n = 21, mean age = 27.0 +/- 5.0 years) participated in two 6-day laborat ory sessions. Each session included one adaptation night, two baseline nigh ts, two consecutive 8-h night shifts followed by 8-h daytime sleep episodes and one recovery night. Subjects took 1.8 mg sustained-release melatonin 0 .5 h before the two daytime sleep episodes during one session, and placebo before the daytime sleep episodes during the other session. Sleep was recor ded using polysomnography. Sleepiness, performance, and mood during the nig ht shifts were evaluated using the multiple sleep latency test (MSLT) and a computerized neurobehavioral testing battery. Melatonin prevented the decr ease in sleep time during daytime sleep relative to baseline, but only on t he first day of melatonin administration. Melatonin increased sleep time mo re in subjects who demonstrated difficulty in sleeping during the day. Mela tonin had no effect on alertness on the MSLT, or performance and mood durin g the night shift. There were no hangover effects from melatonin administra tion. These findings suggest that although melatonin can help night workers obtain more sleep during the day, they are still likely to face difficulti es working at night because of circadian rhythm misalignment. The possibili ty of tolerance to the sleep-promoting effects of melatonin across more tha n 1 day needs further investigation.