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For decades we've been hearing high school teachers say that their students sit through first- and second-period classes weary and bleary eyed. But only in recent years have school districts started to change, or seriously consider changing, school starting times. This nationwide movement was prompted at least in part by studies on the circadian rhythms of adolescents. These studies, conducted primarily by the research group of Mary Carskadon, professor of psychiatry and human behavior at the Brown University School of Medicine and director of sleep and chronobiology research at E.P. Bradley Hospital in Rhode Island, have shown that puberty—the transition from childhood to adulthood—may cause a shift in a child's natural sleep-wake cycle. This shift means that adolescents will stay awake late at night and, if given the opportunity, will wake up late. As children go through puberty, a common trend is that they start to go to sleep later at night and, as a result, to sleep less. However, this is not because teenagers need less sleep. According to Carskadon's research, teenagers continue to need about 8.5 to 9.25 hours of sleep each night (compared with the 7.5 to 8 hours needed by adults). Teenagers sleep less because many external factors—social events, piles of homework, sports, television, and evening jobs—are keeping them from going to bed early. In recent years, it has become apparent that internal biological processes also influence the later timing of adolescent sleep. The first evidence that biology may cause a shift in the adolescent sleep-wake cycle came from a 1993 study that looked at a group of 275 sixth-grade girls. Researchers asked a series of questions to estimate each girl's physical development and determine her tendency to favor activities earlier or later in the day. The results showed that more physically mature girls favored activities later in the day, whereas less mature girls favored activities earlier in the day. A subsequent study by Carskadon's group looked at changes in melatonin secretion in 19 adolescents. Melatonin is a hormone produced by the brain's pineal gland during the night; production turns on in the evening and off in the moring. For this study, Carskadon developed a unique experimental protocol designed to control for behaviors, such as a later bedtime and hence later exposure to light, that would affect the circadian timing system. Study participants were kept on identical schedules at their homes and then in the laboratory. Melatonin samples were collected from saliva at specific times during the day and night. The study found that melatonin production turned off later in more mature adolescents, presumably indicating that the brain's nighttime cycle shifted to a later phase. The conclusion from these and other studies is that pubertal development is associated with a delay in circadian phase and, therefore, a later timing of sleepiness and wakefulness. Carskadon's field studies have also shown that teenagers with a delay in circadian phase are excessively sleepy in the morning hours and, if they take morning naps, their sleep patterns are reversed—that is, REM (rapid eye movement) sleep occurs before non-REM sleep. These are two classic manifestations of sleep disorder. The easiest explanation for this phase delay is that the circadian rhythm of adolescents has a slightly longer period—that is, a longer internal day length. However, this hypothesis has yet to be proved, partly because it is difficult to obtain an accurate measurement of circadian period in humans. Because the circadian clock is reset (entrained) by external cues, such as light, techniques for determining a circadian period typically require complete isolation from the outside world for a month or longer. Not many teenagers (or their parents) would agree to that! Carskadon is currently using a research technique that permits adolescents to be studied in small groups in isolation from external time cues but not from human contact. According to Carskadon, preliminary data show that the internal period of adolescents is longer than that of adults. Carskadon's group is also exploring whether adolescents are more sensitive to low-level light—including the glow from computers—in the evening. An increased sensitivity to light in the evening could affect the circadian timing system by pushing the zone for sleep to a later time. Regardless of mechanism, Carskadon's findings that adolescents need more sleep than adults and that they have a natural tendency to go to sleep late in the evening prompted the Minneapolis Public Schools to delay the starting time of the city's 7 high schools by 85 minutes, from 7:15 a.m. to 8:40 a.m. As a result, according to a preliminary survey by the Center for Applied Research and Educational Improvement at the University of Minnesota, most teachers report having a greater number of students who are more alert and fewer students who fall asleep at their desks. Because of these preliminary results and Carskadon's research findings, other school districts are now considering similar changes. On the national level, Rep. Zoe Lofgren (D-Calif.) has proposed a bill known as the "Zzzz's to A's Act." It would provide up to $25,000 to school districts as an incentive to change school starting times.
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