Many teenagers find it hard to get up in the morning. This is not because they are lazy. During the teenage years, the body clock moves later. The sleep hormone starts later at night, so teens do not feel sleepy early.

Scientists found that people become most "night owl" around the age of 20. In Seattle, schools started about one hour later, and students slept 34 minutes more.

Teenagers who cannot get up in the morning are often called lazy, but research shows that their bodies are changing. During adolescence, the brain starts releasing melatonin, the hormone that signals bedtime, later at night. In addition, sleep pressure seems to build up more slowly during the day, so teens do not feel sleepy until late.

A 2004 survey in Germany found that people become more and more of a night owl through their teens and reach a peak around the age of 20. After that, they gradually become morning types again. In 2016, high schools in Seattle moved their start time from 7:50 to 8:45. Students then slept a median of 34 minutes more, and their grades improved.

Adolescents who struggle to wake up are frequently accused of laziness, but sleep research points to biological change. During puberty, the evening release of melatonin, the hormone that signals the onset of night, shifts later, and experiments suggest that sleep pressure accumulates more slowly during waking hours. Together, these changes make it hard for teenagers to feel sleepy at a time that suits early school schedules. A large 2004 German study based on questionnaires found that people grow increasingly evening-oriented through adolescence, reaching a peak at around 20, slightly earlier in women than in men, before gradually becoming morning types again.

The researchers proposed that this peak might serve as a marker for the end of adolescence. In 2014, the American Academy of Pediatrics recommended that middle and high schools start no earlier than 8:30 a.m. When Seattle high schools delayed their start by 55 minutes in 2016, students wearing activity monitors slept a median of 34 minutes longer on school nights, and median grades rose by 4.5 percent. Because the study compared different groups of students before and after the change rather than randomly assigning them, other factors may have contributed.