Many parents believe teenagers stay up late because they are lazy or because of their phones. Scientists say the real reason is inside the body. During the teenage years, the internal clock moves later by about two hours. A boy of fourteen may simply not feel sleepy at ten in the evening, even when he is tired.

The body prepares for sleep by making a chemical called melatonin. In children this starts early in the evening, but in teenagers it starts much later. So a teenager who lies in bed at ten may still be awake at midnight. The next morning the alarm rings at six, and the result is a short night.

Losing sleep affects more than mood. Tired students find it harder to remember new words, to solve problems and to control their feelings. Teachers often notice that the first lesson of the day is the quietest one, and not in a good way.

Some schools have tried a simple change: starting classes an hour later. In these schools, students slept longer, arrived on time more often and reported feeling better. But a later start is difficult for families, because parents leave for work early and school buses follow fixed timetables. So many schools instead teach students about light: bright screens late at night delay melatonin, while morning sunlight helps the clock move earlier again.