Figure 1. Diagram of a laboratory session.

Subjects participated in two laboratory sessions. They slept on a fixed sleep schedule tailored to their habitual sleep times for 7 days before the first laboratory session and for 9 days in between the two laboratory sessions. During the phase assessments, on days 1 and 5, saliva was sampled every 30 min in dim light (< 5 lux) to measure the entire melatonin profile. There were three days of an ultradian light–dark cycle during which subjects were permitted to sleep in the dark (0 lux) for 1.5 h (represented by black bars) and were kept awake for 2.5 h in room light (< 150 lux). Each day subjects were given a pill at the start of one of the awake episodes, and thus at the same time of day for 3 days in each laboratory session. During one laboratory session the pill was melatonin (3 mg) and during the other session it was placebo, counterbalanced. Different groups of subjects received the pills at the start of different awake episodes in order to cover all 24 h. The phase shift (free-run) during the placebo session was subtracted from the phase shift during the melatonin session to generate the PRC.