Counting sleep

President Bill Clinton boasted that he required just four hours of shut-eye a night. Martha Stewart reportedly bakes and decorates on four to five hours, while inventor Thomas Edison spent just three or four hours in bed, believing sleep was wasted time.

Well, as it turns out, some - but just some - of these super-productive types might be right.

A growing body of research suggests that certain people may be able to withstand sleep deprivation better than others, and that the ability to perform relatively well on little sleep - at least for a short period - could be an inherited trait, like eye color and height. At least two laboratories have reported discovering genes that might bestow an ability to thrive on less-than-average sleep, and more are searching for such genes.

“Some people tolerate a lack of sleep, while others fall apart,’’ said Dr. Christopher Landrigan, a sleep researcher at Brigham and Women’s Hospital in Boston.

Researchers have found, however, that just because a person thinks he or she can function on little or no sleep does not necessarily mean that it’s true. During studies in sleep labs, some people who claim they need only a few hours a night did poorly on performance tests, and vice versa.

“There is a huge discrepancy between self-assessment and how alert people actually are,’’ said Dr. Elizabeth Klerman, who also studies sleep at the Brigham.

Overall, though, there appear to be significant differences between people, differences that employers - including hospitals, the military, and NASA, which can require long hours without sleep - are interested in learning how to predict, so they can tell who will perform best with less sleep and who might need extra help.

Researchers have studied sleep for decades, focusing in particular on the optimal amount and timing of sleep. But since study subjects must spend days - sometimes weeks - in a lab outfitted with a bed and constant monitoring, the studies are expensive and recruiting subjects is difficult. In the last five years, researchers have accumulated enough data to begin comparing individuals, while at the same time genetic research has taken off.

Interest in sleep also may have grown because many Americans are getting less of it. From 1998 to 2005, the number of US adults who reported getting 8 or more hours of sleep a night on weekdays fell from 35 percent to 26 percent, according to the National Sleep Foundation, a nonprofit research organization in Washington, D.C.

As a result, researchers are studying not only what happens to people when they are completely deprived of sleep for 24 or 36 hours, but also the impact of “restricted’’ sleep - getting five hours a night for several weeks.

To do this, scientists test study subjects in the lab on memory, mood, reaction time, and critical thinking skills. Very little research has measured how sleep-deprived individuals perform in real life.

Early estimates suggest that 10 to 15 percent of people function relatively well on little or no sleep, while another 10 to 15 percent fall apart, Landrigan said. For example, in one test following sleep deprivation, researchers flash numbers on a computer screen to measure study subjects’ reaction time. As soon as the numbers start to roll off the screen, subjects are instructed to press a button. The average person who has had a full night’s sleep presses the button in a quarter-second, Landrigan said. After one night without sleep, some people are able to maintain this speed. But others will be “completely glazed over’’ and take 4 to 6 seconds to press the button, he said.

After 48 hours without sleep, just about everyone “will fall apart,’’ he said.

Some people associated with professions famous for long hours, such as medical residency, have justified the workloads by suggesting that the vocations attract particularly resilient people. But informal analysis of sleep studies involving residents, Landrigan said, shows they mirror the general population in terms of the number of people who are able to perform well without much sleep.

Unfortunately, unless you are tested in a sleep lab for at least two days, there is no sure way to tell which group you fall into.

“If you are someone who has found you repeatedly have trouble whenever you stay awake or routinely fall asleep when driving, then it’s prudent to avoid any sleep deprivation,’’ said Dr. David Dinges, chief of the Division of Sleep and Chronobiology at the University of Pennsylvania School of Medicine. His research has shown that people consistently overestimate their alertness.

Klerman has compared people who say they need five or six hours of sleep a night to people who say they need eight, nine, or even 10 hours. She found that regardless of how little or how much sleep people say they need, when they stay in bed 16 hours a day in the lab, they all sleep similar amounts: at least eight hours.

These results suggest that the difference may not be in how much sleep people need, but in how much of a sleep deficit people are able to endure. “Some people are more willing to walk around tired than other people,’’ she said.

Still, many researchers are pushing to find a biological reason for these differences. Scientists at the University of California-San Francisco reported in August in the journal Science that they found a mutated gene that allowed two members of an extended family to thrive on six hours a night.

The law forbids employers from discriminating against workers based on genetics, but some worry where all the research is headed.

Nancy Wesensten, a researcher at Walter Reed Army Institute of Research in Maryland, said she knows of no plans by the military to use this type of information in hiring or promotion decisions, but rather to better manage people’s assignments. Some soldiers might require more sleep than others before an overnight mission, for example. Her lab is using MRI and other imaging tests to look for brain activity that might predict a resilience - or vulnerability - to sleep loss.

“Ideally you want to be able to predict ahead of time how well an individual will perform,’’ Wesensten said. “At some point we’ll have the answers, but we don’t have them yet.’’