Why We Sleep

Besides, the findings don't take into account the quality of sleep you get. Although surveys suggest that we get less sleep than folks did a century ago, that's not necessarily a problem. "Our sleeping environments are better than they ever have been," says Jim Horne, director of the Sleep Research Center at Loughborough University in England. In Victorian workhouses, to give just one example, folks used to sit on benches and drape themselves on long ropes, called hang-overs, to sleep. They must have got used to it, Horne says. Indeed, the sleep system can be very flexible and adapt quickly to different conditions. "It's peace of mind rather than physical comfort that counts anyway," says Horne.

So, how much sleep should you get? Most researchers take a decidedly practical stance. "If you feel sleepy the following day," says Dr. Pierre-Hervé Luppi at the University of Lyons in France, "if you have episodes of sleepiness or a feeling of major fatigue throughout the day, it means you're not sleeping enough." You don't have to know what sleep is for to know that it's good for you. --With reporting by Dan Cray/Los Angeles, Simon Crittle/New York, Helen Gibson/ London and Grant Rosenberg/ Paris