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.
