Fig. 4.

Exposure to room light elicits strong suppression of melatonin during the usual hours of sleep. A, Participants lived in ambient room light (<200 lux) and slept in darkness for 3 baseline days, after which they underwent a constant routine procedure in room light (<200 lux). Data are shown from either d 6–9 or 10–13 of a 14-d research protocol (See Subjects and Methods). B, The melatonin rhythm is shown for five individuals exposed to room light (○) during the constant routine vs. darkness during sleep (●) on the preceding day. Percent melatonin suppression by room light is indicated at the top of each plot for the 8 h corresponding to habitual sleep (enclosed by the vertical dashed lines).