WASHINGTON (AP) -- Scientists have discovered a gene that helps a mother and daughter stay alert on about six hours sleep a night, two hours less than the rest of their family needs.

It's believed to be a very rare mutation, not an excuse for the rest of us who stay up too late. But the finding, published in Friday's edition of the journal Science, offers a new lead to study how sleep affects health.

The National Institutes of Health says adults need seven hours to nine hours of sleep for good health. Regularly getting too little increases the risk of health problems, including memory impairment and a weakened immune system. A major 2006 study estimated that as many as 30 million Americans suffer chronic insomnia, and millions more have other sleep disorders, including sleep apnea.

University of California, San Francisco, researchers have long hunted genes related to how and when people sleep. In 2001, they discovered a mutation that puts its carriers' sleep patterns out of whack: These people regularly go to bed around 7:30 p.m. and wake around 3:30 a.m.

Now the same team has found a gene involved in regulating length of sleep. In one family, the 69-year-old mother and her 44-year-old daughter typically go to bed around 10 p.m., and Mom rises around 4 and her daughter around 4:30, with no apparent ill effects. The rest of the family has typical sleep patterns.

Blood tests showed the women harbored a mutation in a gene named DEC2 that's involved in regulation of circadian rhythms, the body's clock. A check of more than 250 stored DNA samples didn't find another carrier.

Then lead researcher Ying-Hui Fu, a neurology professor, and colleagues bred mice and fruit flies that carried the mutation. Sure enough, the flies' activity and brain-wave measurements on the mice showed those with the mutation slept less -- and the mice needed less time to recover from sleep deprivation.

The result: A model that "provides a unique opportunity" to study the effects of different amounts of sleep, Fu concluded.

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