Fig. 2. Delayed school start times result in later exposure to light in the morning but not in the evening.

(A) Mean student light exposure waveforms between years for school and nonschool days. During school days, students appear to have a delay in morning light exposure but not in evening light exposure. This delay is not evident in the data from nonschool days. (B) For both years, exposure to light is delayed in weekends relative to weekdays. (C) Because of the non-normal nature of the light data, the times for first and last exposure to 50-lux light on school, and nonschool days were tested for each year using a two-way ANOVA. There was a significant effect of day of week (school or nonschool) and year but not of the interaction (see Table 1); ****P < 0.0001, significant difference between years (Sidak’s multiple comparisons). No difference was observed on nonschool days nor in the timing of the last daily exposure for school or nonschool days.