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How much sleep do you need? The answer may be found in your genes.

U.S. researchers have identified a genetic variant that allows some people to get by on very little sleep. The discovery, published in the journal Science, is based on a genetic analysis of a mother and daughter who normally have just six hours of shut-eye a night. The gene is believed to play a role in regulating circadian rhythm, essentially the body's built-in clock.

The lead researcher, Ying-Hui Fu of the University of California, San Francisco, expects more sleep genes will be found. After all, some people seem to need nine or more hours of sleep a night.

Medical experts don't yet know if it's healthier to be a short- or long-duration sleeper. But Dr. Fu noted that the short-duration sleep gene also has been pinpointed in mice. "We will be able to use these mice to do a controlled study to address these questions," she said in an e-mail.