One reason that adolescents become night owls is simply due to the demanding lifestyle of this age group -- which includes heavy loads of homework and busy social lives.
But another, more natural reason is triggered by a unique biological shift that occurs during the teenage years.
Scientists have known for a long time now that our biological clocks shift forward during adolescence! Instead of feeling drowsy in the evenings, teenagers tend to become more alert and have a difficult time settling into sleep (likely because melatonin, which causes sleepiness, is secreted on a delayed timetable).
By the same token, in the mornings -- when young children and adults are wide-awake and primed for the day -- teenagers maintain elevated melatonin levels, and often feel groggy as a result.
And this is exactly why the chorus of doctors and school administrators advocating for later high school start times has grown louder in recent years.