Objective Examine, in a field study, circadian phase changes associated with two different light–dark exposures patterns, one that was congruent with a phase advanced sleep schedule and the other that was incongruent with an advanced schedule. Methods Twenty-one adults (mean age ± SD = 22.5 ± 3.9 years; 11 women) participated in the 12 day study. After a five day baseline period, participants were all given individualized, fixed, 90 minute advanced sleep schedules for one week. Participants were randomly assigned to one of two groups, an advance group with a light–dark exposure prescription designed to advance circadian phase or a delay group with light–dark exposure prescription designed to delay circadian phase. The advance group received two morning hours of short-wavelength (blue) light (λmax ≈ 476 ± 1 nm, full-width-half-maximum ≈ 20 nm) exposure and three evening hours of light restriction (orange-filtered light, λ < 525 nm = 0). The delay group received blue light for three hours in the evening and light restriction for two hours in the morning. Participants led their normal lives while wearing a calibrated wrist-worn light exposure and activity monitor. Results After seven days on the 90 minute advanced sleep schedule, circadian phase advanced 132 ± 19 minutes for the advance group and delayed 59 ± 7.5 minutes for the delay group. Conclusions Controlling the light–dark exposure pattern shifts circadian phase in the expected direction irrespective of the fixed advanced sleep schedule.

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