Objective: To quantify circadian rhythms in rest-activity cycles in depressed children and adolescents. Method: Restactivity cycles were evaluated by actigraphy over five consecutive 24-hour periods in 100 children and adolescents, including 59 outpatients with major depressive disorder (MDD) and 41 healthy normal controls. Total activity, total light exposure, and time spent in light at more than 1,000 lux were averaged over the recording period for each participant. Time series analysis was used to determine the amplitude and period length of circadian rhythms in rest-activity. Results: Overall, adolescents with MDD had lower activity levels, damped circadian amplitude, and lower light exposure and spent less time in bright light than healthy controls. Among children, those with MDD showed lower light exposure and spent less time in bright light, but only depressed girls showed damped circadian amplitude. The sex differences were substantially greater in the MDD group than in the normal control group. Conclusions: These results confirm damped circadian rhythms in children and adolescents with MDD and highlight the influence of gender and age on these measures. J. Am. Acad. Child Adolesc. Psychiatry, 2004;43(6):761-769. Key Words: rest-activity cycles, circadian rhythms, childhood depression, gender.