Controlling light-dark exposure patterns rather than sleep schedules determines circadian phase

Objective: To 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 one that was incongruent with an advanced schedule. Methods: Twenty-one adults (mean age±standard deviation=22.5±3.9years; 11 women) participated in the 12day 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 (λ_max≈476±1nm, full-width-half-maximum≈20nm) exposure and three evening hours of light restriction (orange-filtered light, λ<525nm=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.