Sleep is not only regulated by the body clock, but also by how long we were awake (also known as the buildup of “sleep pressure”). But not all waking hours are equal. We’ll get more tired skiing, for example, than sitting at a desk sending e-mail. This is one reason we sometimes lie awake at the end of a long day at the office despite utter exhaustion.
Think of teenagers as early shift-workers who suffer the most social jet lag. They go to school at their biological equivalent of midnight with profound consequences for learning and memory. They suffer from sleep deprivation during the school week and certainly should be allowed to catch up on weekends. However, they should sleep with daylight coming into their bedrooms and should refrain from using light-emitting devices after 10 p.m.