Data have been presented by several investigators demonstrating a shift toward later bedtimes in teenagers. These data have practical implications for sleep deprivation, daytime sleepiness, school performance, and driving safety. However, it is not entirely clear from the human data to what extent later bedtimes are a consequence of changes in life-style (such as release from parental bedtime regulations, peer socialization activities, and part-time jobs) and to what extent they reflect a biologically driven maturation process. If biological maturation leads to the ability to stay up late or the ability to adhere to earlier bedtimes, then we can hypothesize that developmental trends in circadian activity are tied to other aspects of maturation during adolescence and will be disrupted by interventions that generally interfere with adolescent maturation. Data in this chapter suggest that adolescent trends in later sleep onset occur in nonhuman primates and can be disrupted by zinc deprivation, a nutritional intervention that leads to retardation of adolescent growth, skeletal, and sexual maturation. This is a new piece of information supporting the biological origin of altered bedtimes during adolescence. The background for the study is first presented, followed by data indicating that shift in the offset of daily activity normally occurs in rhesus monkeys during the period of adolescent maturation and that it can be prevented by zinc deprivation.