Backgrounder: Later School Start Times
adolescents became sleepier in the middle of the day. According to the tests, more mature adolescents showed signs of reduced alertness during the day even though they slept an equivalent amount at night (Carskadon et al., 1980).
Changes in Melatonin
Another experiment, conducted by Dr. Mary A. Carskadon of Brown University, found that more mature adolescents had later circadian rhythm timing, based on melatonin secretions in saliva samples. This finding shows that melatonin secretion occurs at a later time in adolescents as they mature; thus, it is difficult for them to go to sleep earlier at night. The melatonin secretion also turns off later in the morning, which makes it harder to wake up early (Carskadon et al., 1998).
Another important finding from many studies is that the circadian timing system can be reset if light exposure is carefully controlled (Carskadon et al., 1997). In studies where adolescents are paid to keep a specific sleep schedule and wear eyeshades to exclude light during evening hours, measurements of melatonin secretion show that the rhythm had moved significantly toward a designated time. This means that with time, effort, and money, researchers can get adolescents to reset their clocks. This approach, however, is not necessarily realistic for teens who have full and busy lives. Nevertheless, the interaction of light exposure and sleep timing is important to keep in mind.
A Widespread and High-Impact Part of Teens' Lives
Findings of the tendency for adolescent sleep patterns to be delayed have been reported not only in North America, but also in South America, Asia, Australia and Europe (Andrade & Menna Barreto, 2002; Carskadon & Acebo, 1997; Ishihara, Honma & Miyake, 1990; Bearpark & Michie, 1987; Strauch & Meier, 1988; LeBourgeois et al., 2005; Thorleifsdottir et al., 2002). The diversity of such research supports the view that intrinsic developmental changes play a role in delayed sleep patterns in adolescents. This biological shift sets the stage for other social and environmental conditions that make it easier for these adolescents to stay awake at night and wake up sleepdeprived. The effects of changing sleep patterns are compounded by