If you have a teenager (or remember your high school sleep habits) you know that staying up late, struggling to rise with the alarm clock, and dozing until noon on the weekends come with the territory. One of the reasons adolescents become night owls is the lifestyle of this age group—which brings heavy loads of homework and busy social lives—but another reason is a natural, biological shift that occurs in the teenage years.
Scientists have known for a long time now that a person's biological clock shifts forward in adolescence. Instead of feeling drowsy in the evening, teenagers actually tend to become more alert and have a difficult time settling in to sleep (likely because melatonin, which causes sleepiness, is secreted later). In the morning, when people of other ages are awake and primed for the day, teenagers still have elevated melatonin levels and often feel groggy as a result. Many teens also feel drowsy in the middle of the day, regardless of their sleep habits.
That's why many doctors and school administrators have advocated for later high school start times. A teen who rises for school at 6:30 a.m. is fighting against a biological force of sleepiness, and later might find it hard to doze off with sufficient time to get the roughly nine hours of sleep that adolescents require.
When schools shift their schedules, teens benefit. For example, seven high schools in Minneapolis moved their start times from 7:25 a.m. to 8:30 a.m. and tested the outcomes for their students. As a result of the change, the teens got five or more extra hours of sleep per week, and attendance and enrollment rates went up, as did alertness. Meanwhile, student-reported depression went down. This proves true for middle schoolers as well. In a study of 7th and 8th graders, those who went to a school that started at 8:37 a.m. reported sleeping an hour more on school nights. The ones who started at 7:15 a.m. were tardy four times as often and had significantly worse grades, both in the fall (when the start time changed) and again in