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Based on human and animal research, it appears that light administered in the last two hours of sleep might be particularly effective as an antidepressant and in advancing circadian rhythms. Green light might be more effective than white light. An obvious advantage of light treatment during sleep is that disturbance and time cost during waking hours might be avoided. For these reasons, we decided to explore effects of a light treatment with a green light mask used for in the last hours of sleep. Subjects were 30 adults aged 18-35 years. This protocol took take place at the subjects' homes and lasted 15 days after recruitment (3-day baseline period and 12-day intervention period). Throughout the study, participants maintained sleep and compliance diaries and wore a wrist actigraph. The study was a randomized, controlled clinical trial of a bright green light mask versus a dim red light mask. The two treatments were a bright green light and dim red light. Post-treatment interviews showed that all subjects reported some discomfort from the masks, but compliance was good overall. Symptom inventory scores did not differentiate groups, suggesting no significant differences in side effects. Mood ratings and sleep questionnaires did not differentiate groups. Sleep diaries distinguished groups, suggesting worse sleep overall in the green mask group. Actigraphy showed a significant circadian phase advance in the green light group. This suggests that the green light had a physiological effect. These findings suggest that green light masks are safe and tolerable; while the green light mask worsened perceived sleep overall, no specific factors were implicated and such complaints may simply be due to earlier awakening caused by a phase advance. Also, early morning green light presented during sleep produced a significant phase shift, suggesting that while there were no symptom benefits evidenced in this healthy sample, green light masks may be useful in clinical and research applications (e.g., mood disorders, endocrine functioning, delayed sleep phase syndrome) where circadian phase shifts have resulted in positive outcomes