1. Bright light exposure has been demonstrated as an effective treatment for circadian rhythm sleep disorders. Recent studies suggest that more moderate intensities of light might affect endogenous rhythms. A light mask treatment, using light applied through eyelids during sleep, was tested for Delayed Sleep Phase Syndrome. 2. The active light group (n = 5) received 500 lux light for 3 hours prior to awakening for 12 days. The placebo light group (n = 5) received 0.1 lux light with the same timing. Circadian rhythm phase was assessed from core body temperature and urinary 6-sulfatoxymelatonin measurements. The SIGH-SAD-SR mood scale was administered to assess mood. 3. There were slight trends toward a phase advance of the body temperature rhythm and a phase delay of the melatonin rhythm, and both groups reported anti-depressant benefits. However, no statistically significant effects of 500 lux light mask treatment were demonstrated compared with the placebo-light treatment. 4. More extensive studies will be required to clarify the factors of dose-response and phase-response.