Children change during puberty, so do their sleep habits. Specifically, their biological rhythms — the sleep and wake cycle — get longer to varying degrees. This can cause a teen to function better at night and need to sleep later.
Adolescents with very long rhythms — 25 hours or longer — are considered delayed. When this happens, the body schedule can conflict with that of school and of society, and the teen can experience daytime sleepiness or another common symptom, hyperactivity.
CNN news article here
