Fig. 2.

Exposure to room light before bedtime shortens melatonin (MLT) duration. A, Histograms show the timing of melatonin onset (gray bars) and offset (white bars) in participants (n = 104) living in room light vs. dim light. White and gray bars at the top of each plot indicate exposure to room light (<200 lux) and dim light (<3 lux), respectively, and black bars indicate scheduled sleep in darkness. B, Histograms show melatonin duration in the same participants over three consecutive cycles corresponding to A. Median melatonin duration is indicated by the vertical line with label. Melatonin duration is longest when the onset and offset occur under dim light. C, Horizontal bar chart showing the change in timing of melatonin onset in individual participants from d 2 in room light to d 3 in dim light. In 99% of individuals, melatonin onset occurred earlier in dim light relative to room light. Data are ranked in ascending order of magnitude. D–F, Similar plots are shown for changes in the timing of melatonin offset from the morning of d 3 in room light to d 4 in dim light (D), melatonin onset from d 3 in dim light to d 4 in dim light (E), and melatonin offset from the morning of d 4–5 in dim light (F).