Fig. 1.

Melatonin onset occurs later in room light than in dim light. A, In study 1, participants lived in room light (<200 lux) and slept in darkness for the first two baseline days. Upon awakening on the morning of d 3, participants were exposed to 8 h of room light, followed by 8 h of dim light (<3 lux) before bedtime. After sleep, individuals underwent a constant routine procedure in dim light. B, The melatonin rhythm is shown for a representative subject over three consecutive cycles (d 2–5). In this subject, melatonin onset occurred about 2 h earlier on d 3 and 4 in dim light, compared with d 2 in room light. The timing of melatonin onset and offset are shown by the labeled arrows in gray. White and gray bars at the top of each plot indicate exposure to room light and dim light, respectively, and black bars indicate sleep in darkness.