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