PROJECT SUMMARY A majority of older adolescents in the U.S. are chronically sleep deprived, getting 1- 2 h less sleep than recommended. The circadian (~24-h) system shifts later (delay) with the progression of puberty; this shift contributes to late sleep onsets in older adolescents. Early school start times force teenagers awake earlier than their spontaneous wake time. Thus, the opportunity for sleep shortens. Many adolescents are also waking for school at the “wrong” circadian time. Chronic circadian misalignment and sleep restriction are at their peak during late adolescence, and are associated with morning daytime sleepiness, poor academic performance, conduct problems, depressed mood, suicidal ideation, substance use, insulin resistance, and obesity. Bright light exposure from light boxes can shift rhythms earlier (phase advance) to facilitate earlier sleep onset, and reduce morning circadian misalignment and the associated risks. We constructed the first phase response curves (PRCs) to bright light in older adolescents. To phase advance circadian rhythms, our PRCs showed that the ideal time to begin light exposure was slightly before wake-up time and light should be avoided around bedtime because this is when light produces maximum phase delay shifts. An unexpected finding from our results, however, was a second advancing region in the afternoon (~6 to 9 h after habitual wake-up time) suggesting that afternoon light may have more circadian phase advancing ability than traditionally thought. The overall goal of this mechanistic study is to follow-up on our unexpected PRC findings and test whether individually-timed afternoon light alone and in combination with morning bright light can shift circadian rhythms earlier in older adolescents aged 14 to 17 years. Four groups will be compared in a randomized parallel group design: afternoon bright light, morning bright light, morning + afternoon bright light, and a dim room light control. Adolescents will complete a 2-week protocol. After a baseline week with a stable sleep schedule, adolescents will live in our laboratory for 4 days. Sleep/dark and the time of bright light exposure will gradually shift earlier. Bright light (~5000 lux) will be timed individually based on his/her stable baseline sleep schedule. The first 3-h morning bright light exposure will begin 1 h before wake on the first morning. The first 3-h afternoon bright light exposure will begin 6 h after wake. The morning + afternoon exposures will begin at the same times, but each exposure will be 1.5 h so that a total of 3 h of bright light per day will be given to each group except the dim light control group. Phase shifts of the circadian clocks marked by the dim light melatonin onset (DLMO) is the main outcome. We hypothesize that afternoon bright light will advance DLMO more than dim room light and afternoon bright light will work synergistically with morning bright light to produce larger shifts than morning or afternoon brigh...