Circadian phase shift responses to light (h ± SEM).

Circadian phase shift responses are shown for 6 h of exposure to continuous red light, intermittent red light and darkness, or bright white light near the onset of melatonin secretion. By convention, negative values indicate phase delay shifts. Using a linear mixed-effects model for comparing phase resetting responses, bright white light elicited a larger response than either red light condition (P<0.003). Phase shifts were similar in response to continuous versus intermittent red light (P = 0.69), and did not differ across physiologic measures (P = 0.35). Data were also analyzed using one-way ANOVA, whereby asterisks (*) indicate significant differences in response to bright white light versus continuous red light, and daggers (†) indicate significant differences in response to bright white light versus intermittent red light. Phase resetting did not differ between red light conditions.