Teenagers do not simply choose to stay up late. During adolescence the body clock shifts, so the hormone that brings on sleep is released one to two hours later than in childhood, and it stays late until the early twenties. A school day beginning at half past seven therefore asks most teenagers to be awake and thinking at a time their bodies still treat as night. Telling them to go to bed earlier does not help, because the clock, not the bedtime, is what has moved.