The scientists were searching the samples for variations in several genes thought to be related to the sleep cycle. In what amounts to finding a needle in a haystack, they spotted two DNA samples with abnormal copies of a gene called DEC2, which is known to affect circadian rhythms. They then worked back to find out who provided the samples and found a mother and daughter who were naturally short sleepers. The women routinely function on about 6 hours of sleep a night; the average person needs 8 to 8.5 hours of sleep.

When scientists bred mice with the same mutation, the animals slept less and recovered quicker from periods of sleep deprivation compared with regular mice.

“We know sleep is necessary for life, but we know so little about sleep,” says Ying-Hui Fu, study co-author and professor of neurology at the University of California, San Francisco. “As we understand the sleep mechanism more and more and all the pathways, we’ll be able to understand more about what causes sleep problems.”

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What distinguishes the two women in the study and other naturally short sleepers is that they go to bed at a normal time and wake up early without an alarm. The two women, one in her 70s and the other in her 40s, go to bed around 10 or 10:30 at night and wake up alert and energized around 4 or 4:30 in the morning, Dr. Fu said.

“When they wake up in morning, they feel they have slept enough,” Dr. Fu said. “They want to get up and do things. They arrange all their major tasks in their morning.”

Dr. Fu said that while many people might sleep only six or fewer hours a night, most were not naturally short sleepers. For instance, they use stimulants and alarm clocks to maintain a shortened sleep schedule.

“Many people get only six hours of sleep a night, but we drink coffee and tea to make ourselves stay up,” she said. “That’s a very different thing. Our body needs 8 to 8.5 hours.”

The genetic mutation appears to be rare. Out of 70 families with known sleep problems studied at the university, only one family carried the mutation. Dr. Fu said fewer than 5 percent of people appeared to be naturally short sleepers.

The real benefit of the research will come if and when the mutation is identified in other individuals. That could lead to new discoveries about sleep timing and duration, and possibly new treatments for sleep disorders.

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Dr. Fu said her “fantasy” was that the finding might eventually lead to a safe treatment for people who wanted or needed more awake hours and were looking for a way to get by on less sleep without harming their health.