The melatonin/placebo intervention days follow the initial CP. On experimental Day 4-5, scheduled sleep will occur 5 hours before habitual bedtime. Melatonin or placebo will be administered 30 minutes before lights-out. Twelve hours after waking the following day (Day 5), the CP to asses final circadian phase will begin and continue for 22 hours. The final sleep episode will be scheduled from 18:00 to 02:00 h to ensure that post-treatment phase estimates are in no way compromised by change of sleep time. Subjects will be informed that they are likely to experience sleep disruption, impaired alertness and other symptoms of jetlag for up to several days after they are discharged from the laboratory. Four hours after the CP on Day 7, the subject will be discharged home.