How much sleep do you really need? Why Seven Hours of Sleep Might Be Better Than Eight

Experts generally recommend seven to nine hours a night for healthy adults. Sleep scientists say new guidelines are needed to take into account an abundance of recent research in the field and to reflect that Americans are on average sleeping less than they did in the past.

Several sleep studies have found that seven hours is the optimal amount of sleep—not eight, as was long believed—when it comes to certain cognitive and health markers, although many doctors question that conclusion.

Other recent research has shown that skimping on a full night's sleep, even by 20 minutes, impairs performance and memory the next day. And getting too much sleep—not just too little of it—is associated with health problems including diabetes, obesity and cardiovascular disease and with higher rates of death, studies show.

"The lowest mortality and morbidity is with seven hours," said Shawn Youngstedt, a professor in the College of Nursing and Health Innovation at Arizona State University Phoenix. "Eight hours or more has consistently been shown to be hazardous," says Dr. Youngstedt, who researches the effects of oversleeping.

The Centers for Disease Control and Prevention is helping to fund a panel of medical specialists and researchers to review the scientific literature on sleep and develop new recommendations, probably by 2015.

Daniel F. Kripke, an emeritus professor of psychiatry at the University of California San Diego, tracked over a six-year period data on 1.1 million people who participated in a large cancer study. People who reported they slept 6.5 to 7.4 hours had a lower mortality rate than those with shorter or longer sleep. The study, published in the Archives of General Psychiatry in 2002, controlled for 32 health factors, including medications.

In another study, published in the journal Sleep Medicine in 2011, Dr. Kripke found further evidence that the optimal amount of sleep might be less than the traditional eight hours. The researchers recorded the sleep activity of about 450 elderly women using devices on their wrist for a week. Some 10 years later the researchers found that those who slept fewer than five hours or more than 6.5 hours had a higher mortality.

Other experts caution against studies showing ill effects from too much sleep. Illness may cause someone to sleep or spend more time in bed, these experts say. And studies based on people reporting their own sleep patterns may be inaccurate.

"The problem with these studies is that they give you good information about association but not causation," said Timothy Morgenthaler, president of the American Academy of Sleep Medicine, which represents sleep doctors and researchers, and a professor of medicine at the Mayo Clinic Center for Sleep Medicine.

Dr. Morgenthaler advises patients to aim for seven to eight hours of sleep a night and to evaluate how they feel. Sleep needs also vary between individuals, largely due to cultural and genetic differences, he said.

Getting the right amount of sleep is important in being alert the next day, and several recent studies have found an association between getting seven hours of sleep and optimal cognitive performance.

A study in the journal Frontiers in Human Neuroscience last year used data from users of the cognitive-training website Lumosity. Researchers looked at the self-reported sleeping habits of about 160,000 users who took spatial-memory and matching tests and about 127,000 users who took an arithmetic test. They found that cognitive performance increased as people got more sleep, reaching a peak at seven hours before starting to decline.

After seven hours, "increasing sleep was not any more beneficial," said Murali Doraiswamy, a professor of psychiatry at Duke University Medical Center in Durham, N.C., who co-authored the study with scientists from Lumos Labs Inc., which owns Lumosity. He said the study replicated earlier research, including a look at memory loss. "If you think about all the causes of memory loss, sleep is probably one of the most easily modifiable factors," he said.

Most research has focused on the effects of getting too little sleep, including cognitive and health declines and weight gain. David Dinges, a sleep scientist at the University of Pennsylvania's Perelman School of Medicine who has studied sleep deprivation, said repeatedly getting just 20 or 30 minutes less than the minimum recommendation of seven hours can slow cognitive speed and increase attention lapses.

Experts say people should be able to figure out their optimal amount of sleep in a trial of three days to a week, ideally while on vacation. Don't use an alarm clock. Go to sleep when you get tired. Avoid too much caffeine or alcohol. And stay off electronic devices a couple of hours before going to bed. During the trial, track your sleep with a diary or a device that records your actual sleep time. If you feel refreshed and awake during the day, you've probably discovered your optimal sleep time.

The new sleep guidelines will be drawn up by a panel of experts being assembled by the American Academy of Sleep Medicine, the Sleep Research Society, an organization for sleep researchers, and the CDC. The recommendations are meant to reflect evidence that has emerged from scientific studies and are expected to take into account issues such as gender and age, says Dr. Morgenthaler, the academy president.

Another group, the National Sleep Foundation, a nonprofit research and advocacy group, also has assembled an expert panel that expects to release updated recommendations for sleep times in January.

These groups currently recommend seven to nine hours of nightly sleep for healthy adults. The National Heart, Lung and Blood Institute recommends seven to eight hours, including the elderly. Most current guidelines say school-age children should get at least 10 hours of sleep a night, and teenagers, nine to 10.

"I don't think you can overdose on healthy sleep. When you get enough sleep your body will wake you up," said Safwan Badr, chief of the division of pulmonary, critical care and sleep medicine at Wayne State University School of Medicine in Detroit.

A study in the current issue of Journal of Clinical Sleep Medicine seemed to confirm that. Five healthy adults were placed in what the researchers called Stone Age-like conditions in Germany for more than two months—without electricity, clocks or running water. Participants fell asleep about two hours earlier and got on average 1.5 hours more sleep than was estimated in their normal lives, the study said.