A hallmark of adolescence is a shift to a delayed sleep phase. Sleep patterns in adolescents are characterized by later bedtime and wake-up times, by curtailed sleep duration during weekdays, and by large differences in sleep timing and duration on weekdays and weekends. In addition to changes in bioregulatory sleep mechanisms, several environmental and psychosocial factors related to culture are involved in changes in adolescent sleep patterns. Environmental factors may include early school start times, increased school workload, employment, and increased electronic media exposure. Psychosocial factors may include stress, depressed mood, overweight, smoking and alcohol consumption. These factors often contribute to reduced sleep duration during weekdays, to erratic sleep schedules and to daytime sleepiness. Decreases in health, daytime functioning and academic performance are possible consequences of poor and inadequate sleep in adolescents.