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Harvard Commentaries
Harvard Commentaries
Reviewed by the Faculty of Harvard Medical School

Medical Myths Medical Myths

Sleep: What's Going on Behind That Shut-Eye?

February 27, 2013

By Robert H. Shmerling M.D.

Beth Israel Deaconess Medical Center

Last reviewed February 27, 2013

Have you ever watched someone sleep and wondered what he or she was dreaming? The person’s outward appearance would never give it away: slow breathing, eyes occasionally fluttering, but mostly the very picture of peace and stillness. But, appearances can be deceiving. When people sleep, there’s a lot more going on than meets the eye. The notion of sleep as the body’s ultimate “down time” has some truth to it, but sleep is also an active process, in which the brain can be remarkably busy, even if the body remains (mostly) immobile.

What’s Going On During Sleep?

There is much about sleep that remains mysterious, but experts divide it into two main categories, based on observations of sleeping persons and recordings of the brain’s electrical activity during sleep:

    • Non-REM sleep — This is divided into four stages, with Stage 1 the lightest and Stage 4 the deepest.
    • Rapid-eye movement (REM) sleep — During REM sleep, dreaming is common, muscles (other than the eyes) are inactive, and electrical activity in the brain is similar to that of an awake person. The blood pressure and heart and breathing rates may suddenly increase for short periods of time, just as they do during wakefulness

During a typical eight-hour period of sleep, a person drifts from wakefulness to Stage 1 non-REM sleep, through Stages 2, 3 and 4 and finally REM sleep over the first several hours. During the last half of the night, REM sleep and Stage 2 sleep alternate for 90 to 120 minutes each. As we age, brief awakenings increase in frequency, while deeper stages of non-REM sleep decrease.

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Why Do We Sleep?

The function of sleep is not entirely clear, but researchers believe that REM sleep is important for solidifying memories, and perhaps for even more critical functions. Rodents completely deprived of REM sleep die after a few weeks. Non-REM sleep, meanwhile, seems to be important in providing a sense of restored energy and ability to concentrate during the day. A number of theories about sleep attempt to explain its role or roles: a restorative for the mind and body in preparation for the day ahead; or a way of reducing energy consumption, to save energy for activities occurring during the day. Some scientists believe that sleep is evolution’s way of improving survival by preventing animals from preying on each other 24 hours a day.

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Consequence of Sleep Deprivation

It is hard to define just how much sleep is normal; different people seem to need different amounts. Sleep experts define “enough” sleep as how long a person would sleep if there were no alarm clocks; that is, left to decide entirely on your own, how long would you sleep? The other way to define it is how long one needs to sleep in order to feel alert and rested the next day. However, duration of sleep is not the only thing that matters — the quality of sleep also matters.

Too little sleep or poor-quality sleep can cause a number of problems, including difficulty with short-term memory, concentration, depression, anxiety, irritability, poor energy and reduced libido. In short, quality of life at work and at home may suffer terribly due to insufficient or poor sleep. All of these problems may resolve when sleep duration increases or sleep quality improves.

One problem related to sleep deprivation deserves particular emphasis: Automobile and truck accidents attributed to sleepy drivers account for thousands of deaths each year.

Although the impact is uncertain in humans, sleep-deprived rodents have reduced immune function and higher susceptibility to infection.

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Sleep Disorders

There are more than 80 individual sleep disorders, but they are divided into 4 main categories:

    • Dyssomnias, in which there is insomnia (difficulty initiating or maintaining sleep), sleepiness during the day, and abnormal sleep-wake timing; examples include sleep apnea and the effects of medications or alcohol.
    • Parasomnias, in which there is abnormal behavior around sleep, but without excessive sleepiness or insomnia; examples include sleepwalking or night terrors.
    • Medical-psychiatric sleep disorders, in which a condition that causes other problems disrupts or impairs sleep; examples include anxiety, depression, Parkinson’s disease, dementia or gastroesophageal reflux disease (GERD).
    • Sleep problems that cannot be clearly separated from normal variation, or for which there is no consensus among experts; examples include pregnancy-associated sleep disorder and sleep hyperhidrosis (excessive and unexplained sweating during sleep).

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Getting a Good Night’s Sleep

There are changes you can make to improve your chances of getting a good night’s sleep. Sleep experts call this “sleep hygiene,” and these measures are routinely recommended for almost anyone complaining of sleep trouble. Some are just common sense, but, unfortunately, many people for whom sleep is a problem do not recognize their importance. Here are some general guidelines to follow:

  • Establish a schedule and stick to it; sleep when you are sleepy but try to get up and go to bed each day at about the same time.
  • Use a fan or other means of creating a steady, soothing sound to drown out other noises.
  • Reduce alcohol and caffeine intake; avoid caffeine after noon and don’t use alcohol as a sedative before bed.
  • Get heavy curtains or shades to block out bright light early in the morning if you are awakening earlier than you’d like.
  • Check your medication list. Because some medicines can interfere with sleep, ask your doctor or pharmacist about the medicines you take.
  • Do not exercise vigorously within several hours of bedtime.
  • Avoid heavy meals or excessive fluids within an hour or two of bedtime.

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The Bottom Line

The importance of sleep is self-evident, yet much remains unknown or uncertain about how we sleep, why we sleep and how to improve sleep. One thing is certain, however: Sleep is not a passive process or a complete “shut down” of the body — many stages of sleep are as active for the mind as being awake. So, the next time you see someone sleeping, keep in mind that though the body may look quiet and peaceful, there is much more to the story.

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Robert H. Shmerling, M.D., is associate physician at Beth Israel Deaconess Medical Center and associate professor at Harvard Medical School. He has been a practicing rheumatologist for over 20 years at Beth Israel Deaconess Medical Center. He is an active teacher in the Internal Medicine Residency Program, serving as the Robinson Firm Chief. He is also a teacher in the Rheumatology Fellowship Program.

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