Why Do We Sleep?

Why Do We Sleep ?

Before you can work on creating better sleep, you need to first answer these two questions: why do we sleep, and why is sleep important?

We understand a considerable amount about the physiology of sleep. However, the answer to one central question - the precise function of sleep and what it does for our brain and body - remains a puzzle.

Even though we spend almost one-third of our lives asleep, there is no one theory about the function of sleep that is accepted by most sleep researchers.

For our distant ancestors, sleep may have served a general survival purpose by forcing them to be inactive at night when their ability to see, find food, and defend themselves against predators was poor. Since our ancestors didn't function well at night, they slept, tucked out of harm's way.

Research on deep sleep suggests that it is a state of profound rest in which the brain and body shut down. Deep sleep seems to serve a major biological restorative function by renewing our physical energy. Most of the blood in our body is directed to the muscles during deep sleep to help replenish muscular energy. Since most of the blood flow is directed to the muscles during deep sleep, there is little blood flow to the brain, which means that our brain is very inactive. Deep sleep also affords a period of time for our immune system to turn on to combat illnesses. This may explain why we seem to sleep more when we are ill.

Importance of Sleep During Sleep Stages

Several factors suggest that deep sleep is essential and is the most vital stage. First, deep sleep occurs first during the nights and is therefore the stage least likely to be missed. Second, unlike the other stages, we recoup or "make up" virtually all the deep sleep lost as a consequence of sleep deprivation. Third, compared to the other sleep stages, we experience the greatest impairment in our physical functioning (such as aching muscles and stiffness upon awakening) when we forfeit deep sleep.

Stage 2 sleep is probably a less potent form of deep sleep and is also involved with restoring physical energy. However, the brain apparently doesn't regard stage 2 as important as it does deep sleep, for it does not attempt to recover Stage 2 after sleep loss.

REM sleep is perhaps the most fascinating yet least understood stage. Dreams were viewed by Sigmund Freud as a window to the unconscious and have long been associated with the processing of emotional material. However, contemporary research intimates that a principal function of REM sleep is to permit us to process and save novel information in memory, particularly information learned over the latter portion of each day, and "how to" or procedural memory for activities such as typing, driving, or scuba diving.

One reason newborns occupy much of the day sleeping is that they must spend a substantial amount of time in REM sleep to store all the new information to which they are exposed during the first few months of life. Newborns spend a greater percentage of time in REM sleep compared to adults, and it has been suggested that, in the womb, the fetus may spend all of its time in REM. Because REM is so prominent early in life, some scientists speculate that it plays an important role in the maturation of the brain.

Just as deep sleep seems to be a required stage of sleep, so too does REM sleep. We know this because if we are deprived of REM, we will try to makeup for it in the form of REM rebound. Nevertheless, the fact that we make up only about half of the REM sleep that we lose as a consequence of sleep deprivation suggests that it is not as crucial to the brain as deep sleep. And other than causing anxiety, irritability, and difficulty concentrating, there is no proof that REM sleep deprivation causes serious psychological or physical health problems.

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Jacobs, Gregg D., PH.D. Say Good Night to Insomnia. New York, New York: Henry Holt and Company, LLC, 1998: 17-19.