Lethargy in teens is often mistaken for laziness or apathy, or a stubborn refusal to go to bed at a "reasonable time" (another example of their famous rebelliousness). In truth it is beyond their control; their body clocks are programmed in a way that is simply out of sync with adults' rhythms. We feel sleepy when levels of melatonin rise in the brain at night – and, in the morning, we will wake up feeling alert once it has dropped below a certain level. For teens, melatonin simply rises and falls later in the day than adults – meaning that they will feel alert and active when their parents are ready to hit the hay, and tired and drowsy in the early morning, when their parents may have already been up for hours. "Virtually no adults will have melatonin left in their brains at nine o'clock in the morning," says Coleman, "but around half of teenagers do.
David Robson, ‘The biggest myths of the teenage brain’, BBC
