March 14, 2003 -- If you think you're getting enough sleep on fewer than six hours a night, you're probably experiencing the latest documented effect of sleep deprivation: the inability to realize just how tired you really are.
Even missing out on the recommend eight hours of nightly sleep for two weeks -- a fate regularly experienced by millions of Americans -- can accumulate to a "sleep debt" equal to total sleep deprivation of two full nights, finds a new study. Average only four hours a night in that time and your brain reacts as though you haven't slept at all for three consecutive nights.
"And the most worrisome part of this is these people don't realize how sleep-deprived they really are," lead researcher Hans P.A. Van Dongen, PhD, assistant professor of sleep and chronobiology at the University of Pennsylvania School of Medicine, tells WebMD. "When people are put through chronic sleep deprivation, there is an initial response where they say, 'OK, this is not optimal but I'll manage.' But after a few days of this, things are much worse than they realize."
In other words, they actually become too tired to realize just how tired they are. They may feel only a little tired, says Van Dongen, but they have slower reaction time, weakened memory, and other thinking impairments.
His study, published in the March 15 issue of Sleep, is the latest to document the growing list of health risks from sleep deprivation. Two months ago, Harvard researchers found that regularly getting too little sleep boosts heart disease risk, while previous research linked consistent sleep loss with a higher risk of diabetes and obesity. In a finding similar to Van Dongen's, Israeli researchers reported two weeks ago that children who got just one hour less sleep a night than their peers scored far worse on various memory and attention tests.
Van Dongen and colleagues evaluated the effect of mild and moderate nightly sleep loss on 48 young and middle-aged adults. After the sleep deprivation, scores on various types of thinking and memory tests were on par with those of people who were totally deprived of sleep for two consecutive nights. Like the Israeli finding, Van Dongen learned that missing just an hour or two on a regular basis can produce devastating results.
"The take-home message is this: Don't rely on your own sense of whether or not you're getting enough sleep. You may very well be chronically sleep-deprived and consider that normal," he tells WebMD. "In some ways, it's similar to people in chronic pain -- they don't realize how much pain they have until it's relieved."
This self-denial may play a key role in many of the 100,000 car crashes each year in the U.S. that result from sleep deprivation. "Another study showed that 50% of the people who caused car crashes did not perceive that they were sleepy immediately prior to the crash," says Mark Mahowald, MD, director of the Minnesota Regional Sleep Disorders Center and a spokesman for the National Sleep Foundation. "So if you talk to people who are sleep-deprived, half of the time they will be driving impaired but do not perceive themselves to be."
Mahowald, professor of neurology at the University of Minnesota Medical School who was not involved in Van Dongen's study, considers these new findings on sleep deprivation very important. "This study clearly shows that mild but chronic sleep deprivation has very serious adverse performance consequences," he tells WebMD. "How many people are sleep-deprived? Just about everybody. Anyone who uses an alarm clock to wake up is sleep-deprived by definition. Their brains would have awakened spontaneously if they have accumulated the amount of sleep they need."
While some people can function fine on fewer than eight hours of sleep, most need to average that amount over time to keep mind and body healthy.
"These findings don't surprise me in the least," adds sleep researcher Teri J. Bowman, MD, assistant professor of medicine at the Nebraska Medical Center and spokeswoman for the American Academy of Sleep Medicine. "Most Americans think they can beat the odds and cram more time into their schedule. In fact, they cannot."
While she and other experts recommend following "good sleep hygiene" -- maintaining a consistent sleep schedule averaging about eight hours nightly -- the fact is, slightly more than half of all Americans sleep less than that. But the good news is that you can actually make up for lost time. If you've missed out on some precious sleep during the week, Van Dongen suggests that you try to catch up over the weekend or whenever time permits.
"Although the mechanisms for recovery sleep are not fully understood, we know that when you give yourself a chance to recover by giving yourself more than enough sleep, you can quickly catch up," he says. "It's only the chronic sleep deprivation that will actually in the end cause major problems. While it's best to try to get eight hours or more a night, every night, if you can't, try to build in a couple of days every now and then to sleep longer."