Americans associate sleep with laziness, but sleep experts say it's impossible for healthy people to overindulge in sleep. Some people may crave more food than they need, but only people who are physically or mentally ill crave more sleep than necessary. And we humans need a lot.

Some people like to say they'll sleep when they die, but study after study shows that sleeping too little will make you half dead when you're awake — less able to learn, perform, remember, react quickly or make good decisions.

That's how we're teaching young people to live. One of the most counterproductive American habits is forcing teenagers to get up at the crack of dawn for school. Tearing themselves out of bed with the jangle of an alarm may help them learn deprivation and sacrifice, but they won't learn as much math, science, literature or history that way.

University of Washington neurobiologist Horacio de la Iglesia says that teens develop a different circadian rhythm from children or adults. Teens actually need about nine hours of sleep, though few get the chance. And once they go through puberty, their bodies want stay up until midnight, and want to get up after 8. To a teen, he says, getting up at 6:30 feels like getting up at 4:30 for a typical adult.

But this is America, where people are deeply suspicious of personal comfort. Let teens get the amount of sleep they want? What will that do to their productivity? De la Iglesia and colleagues had a chance to find out a couple of years ago when schools in the Seattle district changed their start times from 7:50 a.m. to the much more civilized hour of 8:45 a.m.

Following a cohort of students from two schools before and after the change, the researchers equipped their subjects with wrist motion sensors, considered a more accurate way to measure sleep than self-reporting. There's an assumption, he said, "that teenagers are lazy and therefore if you let them sleep in they will go to bed later at night." But that's not what the study found.

The students with the later start times went to bed around the same time they used to, fell asleep around the same time, and on average got 35 minutes more sleep a night. Grades improved in both schools, and in the poorer of the two schools, students were more likely to arrive on time. De la Iglesia says this might have something to do with the parents being more likely to have early-starting jobs, leaving the responsibility for getting to school on time to the kids. The results were published last Wednesday in the journal Science Advances.

One clue to Americans' counterproductive attitude about sleep comes from a recent interview in Boston Magazine with sleep biologist Charles Czeisler of Harvard Medical School and Brigham and Women's Hospital. He said the military used to deprive soldiers of sleep, thinking this would "train" them to handle sleep deprivation in combat — sort of the way exercise can train you to handle physical exertion. But this is crazy. Czeisler compared the practice to starving people to prepare them for a situation where they had inadequate food.

As for schools, Czeisler said in the interview that before 1910, American schools started at 9 a.m., and that education experts vowed that the U.S. should avoid the mistake of the Germans and British, who were starting school at 7 or 7:30. But school start times in the U.S. crept earlier. Which, in Czeisler's opinion, prevents students from achieving what they might be capable of if they got enough sleep.

Imagine the frenzy if someone invented a drug that enhanced performance as much as adequate sleep does. There's no need for this drug though; that's one advantage that can be had free, no prescription required.

Educating people about proper sleep would be one of the most cost-effective ways to improve public health, said de la Iglesia. Unfortunately many people don't recognize that their performance is degraded. Poor self-assessment is another side effect of losing sleep, he said: "People think they are doing fine."

Flam is a Bloomberg Opinion columnist. She has written for the Economist, the New York Times, the Washington Post, Psychology Today, Science and other publications. She has a degree in geophysics from the California Institute of Technology.