Until they're 21, teenagers need eight hours sleep. Yet only 5 percent of high school seniors are getting that a night.

Not enough sleep is one of the causes of young driver car crashes. Setting aside the fact that sleep loss means they're not at full alertness, teenagers are involved in fatal crashes twice as often as older drivers because 'wiring' in the frontal lobe of their brains that isn't yet completely developed can lead to poor decision-making.

This might not convince teenagers who believe in their own invincibility to get to bed in good time. So tell them, sleep loss also means they could get fat. Not enough sleep increases the hormone that stimulates hunger. At the same time, it decreases the one that suppresses appetite. So sleep loss = weight gain.

In school, according to Po Bronson and Ashley Merryman, authors of "NutureShock: New Thinking About Children", if teenagers are tired, they can't retain information so easily because neurons lose their plasticity. So they can't form "the new synaptic connections necessary to encode a memory." The more a student learns during the day, the more sleep is needed at night. Grades suffer. "The performance gap," say the authors, "caused by an hour's difference in sleep was bigger than the gap between a normal fourth-grader and a normal sixth-grader."

Tell any of this to a teenager, they'll probably say that for their studies they can catch up with Cripps Notes and for their weight, subscribe to the Acai Berry Diet. If that's the case, get them to read the Comments on this recent eatWashington blog.