Normalized melatonin concentrations for Experiment 2. Mean ± SEM melatonin concentrations for the seven subjects who completed the second, three-night, within-subjects study. The first two measurements were collected prior to turning on the mask. The last two measurements were collected 30 and 60 minutes after the light mask was energized (except for the dark, control night when the mask remained off for the duration of the experiment). Nocturnal melatonin suppression levels were 25% and 45% after 60-minute light exposures to the LL1 and LL2, respectively.