Getting Enough Sleep? Dream On

　

by William C. Dement & Christopher Vaughan

　

      Is eight hours’ sleep enough to everyone? Have you ever heard of “sleep debt?” The following article warns us of the consequences of sleep deprivation and suggests ways to work off sleep debt.

　

I once had a patient who was so sleep-deprived that she loaded her dirty dishes into the clothes dryer instead of the dishwasher. She turned on the machine, smashing china and glasses.

    Another patient went to great lengths to obtain a 50-yard-line ticket to a crucial San Francisco 49ers play-off game, but was so sleepy that he dozed off in his seat in the first quarter and stayed asleep until the game was over.

    On an everyday level, sleepy people make math errors, break things and become cross with their families, friends and co-workers. Less commonly, they make mistakes with tragic consequences. It is hard to prove how many fatal car accidents are caused by the driver’s falling asleep, but my conviction is that the number is high. Laboratory experiments have confirmed that the sleep-deprived mind is prone to "microsleeps" － lapses of consciousness so brief that the subject may not even be aware of them.

    In one experiment in our laboratory at Stanford University, a volunteer who had only been allowed four hours' sleep the night before had his eyelids taped open (uncomfortable, but it doesn't hurt). He was asked to press a button every time an irregular strobe light flashed. For a few minutes he tapped the switch after each flash, on average every six seconds. Then a bright flash surged into his pupils － but he did nothing.

    "Why didn't you press the switch just now?" we asked.

    "Because there was no flash," the young man replied.

    The machines we used to monitor brain activity showed that at the very moment the light had flashed, the young man had unwittingly fallen asleep, with his eyes wide open, for two seconds. If he had been behind the wheel of a car, those two seconds could have meant disaster.

    Societal pressures to work more and at odd hours have reduced our sleep time over the past century by about 20 percent. Add to the mix our own era's drive to have and do it all － work, family, sports, hobbies － and there is very little time left for rest. I consider sleep deprivation a national emergency.

　

Sleep Debt and the Mortgaged Mind

    Generally, adults need to sleep one hour for every two hours awake, which means that most need about eight hours of sleep a night. Of course, some people need more and some less. Children and teenagers need an average of about ten hours.

    The brain keeps an exact accounting of how much sleep it is allowed. My colleagues and I coined the term sleep debt because accumulated lost sleep is like a monetary debt: it must be paid back. If you get an hour less than a full night’s sleep, you carry an hour of sleep debt into the next day － and your tendency to fall asleep during the daytime becomes stronger.

    During a five-day workweek, if you got six hours of sleep each night instead of the eight you needed, you would build up a sleep debt of ten hours (five days times two hours). Because sleep debt accumulates in additive fashion, by day five your brain would tend toward sleep as strongly as if you'd stayed up all night. From this perspective, sleeping until noon on Saturday is not enough to pay back the ten lost hours as well as meet your nightly requirement of eight； you would have to sleep until 5 p.m. to balance the sleep ledger.

    But for most people it is difficult to sleep that long because of the alerting mechanism of our biological clock.

　

Wide Awake but Not Rested

    An amazingly precise biological clock within us regulates sleeping and waking, and also synchronizes a vast array of biochemical events in our bodies. It is a timepiece of such astonishing precision that people often wake up a few minutes before their alarm clocks go off.

    Most people have two peak times of alertness daily, at about 9 a.m. and 9 p.m. Alertness wanes to its lowest point at around 3 p.m.; after that it begins to build again. This explains why people who have worked hard all day will often start to feel more alert at the same time every evening despite a large accumulation of sleep debt during the day. I believe this “clock-dependent alerting” can often deceive people into thinking they are sufficiently meeting their sleep needs.

    Some years ago I had the chance to observe a striking example of clock-dependent alerting in action. It was during a visit with my daughter, Cathy, who was then a college student. At first, she was almost alarmingly apathetic and seemed to be totally uninterested in my visit. I suggested we take a walk. It was about four o’clock on a sunny late-spring afternoon. In the course of perhaps 20 to 30 minutes, Cathy changed into a talkative, informative, smiling, even vivacious person.

    The explanation for this marvelous transformation: clock-dependent alerting. Even a typically heavily sleep-deprived college student like Cathy will have a late afternoon/evening period of mental arousal. In her case it was able to abolish her fatigue. Nevertheless, my daughter was still carrying around a heavy load of sleep debt.

　

Sleep and Well-Being

    People sometimes ask me if the exact accounting of sleep debt could mean that they are still deprived from those all-nighters years earlier in college. We don't know what happens to sleep debt in the long term, because research has only been able to measure two-week-long periods. You may have paid off those sleep-deprived stretches when you got sick shortly afterward and slept 18 hours at a stretch. Or the brain may lose track of sleep debt accumulated months or years earlier.

    Or, accumulated sleep debt may do long-term damage to your health. In 1959 the American Cancer Society started a massive study, surveying over one million Americans about their exercise, nutrition, smoking, sleep and other habits. After tracking the group for six years, researchers found that short sleep time had a high correlation with mortality: if people had originally reported sleeping less than seven hours a night, they were far more likely to be dead within six years than those who slept an average of seven hours per night.

    After years of further research, the original results still stand: although sleep needs vary, people who sleep about eight hours, on average, tend to live longer.

    Another interesting finding from this survey was the fact that adults who said they slept ten hours or more per night also tended to have shorter lives. We speculate that these self-described long sleepers are more prone to die because they have undiagnosed sleep disorders, like sleep apnea, in which breathing stops for more than ten seconds, possibly hundreds of times a night. This causes sleep to be disrupted repeatedly by short, unremembered awakenings that may create life-threatening health problems.

    Other, more immediate effects of sleep deprivation on health and well-being have been documented by research. Studies have shown that cognitive skills and physical performance are impaired by sleep debt, but mood is affected even more. People who get less than a full night's sleep are prone to feel less happy, more stressed, more physically frail and more mentally and physically exhausted as a result. Lowering sleep debt can make us feel better, happier, more vigorous and vital.

　

Toward a Sleep-Smart Lifestyle

    We are generally very bad judges of our sleepiness. In 1988 my friend and colleague Tom Roth and his team at the Henry Ford Hospital's Sleep Disorders Center and Research Lab in Detroit studied a group of people who specially claimed that day-time sleepiness － a sure symptom of sleep debt － was not a problem. He first sent them to bed for eight hours － a good night’s sleep, most people would agree. Upon awakening, the subjects said they felt fine.

    Yet when their daytime alertness was tested later on, more than 80 percent were not optimally alert. Of those, about 25 percent who said they felt fine were actually so in need of sleep that they posed a danger to themselves or others. Only about two in ten were optimally alert.

    You can't work off a large sleep debt by getting a good night's sleep. You have to make up as much sleep as possible and avoid amassing another large sleep debt by adopting a sleep-smart lifestyle. Here’s how:

    First, establish your average personal sleep requirement in each 24-hour period to maintain a consistent level of alertness. Start with the number of hours of sleep you think you require － eight for most people. Make a point of getting that amount of sleep for several nights and pay close attention to how you feel during the day, especially during lulls on the job, after lunch or while driving.

    If you are getting drowsier on successive days, then you are getting less than your daily requirement and should add 15 to 30 minutes to your sleep time. If drowsiness continues, you can increase sleep time even more, but if you are feeling sleepy only around bedtime, you are probably close to your optimal sleep time.

    Bear in mind that you'll have to take your biological clock into account. If, like me, you are a "lark," or morning person, your strongest period of clock-dependent alerting occurs at this time. I always deal with a need for extra sleep by going to bed early rather than trying to sleep late. An "owl," or night person, however, might not be able to fall asleep early, and would be better off making up sleep debt by rising as late as possible.

    Finally, if you think you have a sleep disorder, do not hesitate to seek professional help. You can find a list of specialists on the Internet. I recommend SleepNet (www. sleepnet. com) and the American Sleep Disorders Association (www. asda. org). The National Sleep Foundation site (www. sleepfoundation. org) also contains useful information on helping you to knit up " the ravell'd sleeve of care, " as Shakespeare once aptly described a good night's sleep.

    (1 695 words)

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