sleep needed remains constant throughout adolescence, a change in the physiological circadian rhythms seems to occur (Carskadon et al., 1993). There appears to be a shift, beginning in mid-adolescence (Tanner Stage 3),4 to a later timing of melatonin secretion, which is related to sleep onset. In addition, some new work suggests that adolescents may also have increased sensitivity to evening light, which would make them more likely to stay awake later (Carskadon, 1997; Carskadon et al., 1997). This pattern is accompanied by a tendency for mid-afternoon sleepiness, which occurs even in those youngsters who get adequate sleep at night, but it is exacerbated in those who get fewer hours of sleep.
Along with these biological changes, social and environmental factors affect the times at which adolescents go to bed and get up, which results in their getting less sleep than they may actually need. Parental regulation of bedtime decreases as children reach adolescence. More than half of 10-year-olds report that their parents set their bedtimes on school-nights; by age 13, only 19 percent report parental control of school-night bedtimes (Anders et al., 1978; Billiard et al., 1987; Petta et al., 1984). Bedtimes get later and later as youngsters age. By age 12, more than 70 percent of youngsters report that they no longer wake up spontaneously in the morning, but must rely on an alarm or parental awakening, which may indicate insufficient sleep.
Although adolescents' bedtimes get later with age, students in most school districts must start their schooldays earlier with age. This results in teens averaging about 7 hours of sleep on school nights, about 2 hours less than they need. Some school districts are experimenting with starting high school an hour later, at 8:30 a.m. Preliminary results suggest that students are more alert, exhibit fewer behavior problems, and earn better grades (R. Weiss, 1997) with a later starting time.
Having a job during the school week may decrease the amount of sleep an adolescent gets. Students who work more than 20 hours per week stay up later and sleep fewer hours per night than do those
Tanner Stages rate maturation and secondary sex characteristics of adolescents. Tanner Stage 1 refers to prepubescent; at Tanner Stage 2, secondary sex characteristics are beginning to be manifest; at Tanner Stage 3, there are significant manifestations of secondary sex characteristics; Tanner Stage 5 refers to the fully mature adolescent.