I. Answer the following questions on the text

Text

We may not give it a moment's thought, but most of us will probably spend the third of our lives asleep. Yet the nature of sleep has puzzled mankind for thousands of years; it is only in the last quarter of a century that researchers have made scientific attempts to investigate this world, trying to find out why some people have enormous difficulty falling asleep, while others find it impos­sible to stay awake. Far from being a passive state, sleep is a remarkably active one. While the sleeper is outwardly calm, the electrical activity of the brain never stops. As the sleep becomes deeper and deeper, the brainwaves become larger and more spread out.

While the sleeper still thinks thoughts and moves occasionally the metabol­ic processes slow down and the heartbeat drops until deep sleep is reached.

Every day every human being experiences two kinds of sleep that alternate rhythmically throughout the entire sleep period. The discovery of the two kinds of sleep occurred almost accidentally at the University of Chicago. In 1952 Dr. Kleitman became interested in the slow rolling eye movements that accompany sleep onset and decided to look for these eye movements throughout the night to determine whether they were related to the depth or quality of sleep. An entirely new kind of eye movement was notices at certain times during night the eyes began to dart about furiously beneath the closed lids.

Dr. Kleitman coined the term "REM" (for rapid-eye-movement sleep) to de­fine the phenomenon he and his colleagues observed. The other kind eventually acquired the name "NREM" sleep. The "NREM" state is often called "quiet sleep", because of the slow, regular breathing, the general absence of body movement, and the slow, regular brain activity. The body is not paralyzed during NREM sleep. The first sleep of the night is always NREM sleep, which must progress through its various stages before the first REM period occurs. REM sleep, which has been called "active sleep" is an entirely different state of exist­ence. At the onset of REM sleep the sleeper's body is still immobile, but we can see small, convulsive twitches of his face and fingertips.

Experts speculate that REM sleep protects us from acting out our dreams and hurting ourselves, that it is not really sleep at all, but a state in which the subject is awake, but paralyzed and hallucinating. The sleeper's breathing be­comes irregular - very fast, then slow - he may even appear to stop breathing for several seconds. If you gently pull back the eyelids the sleeper seems to be actually, looking at something. Cerebral blood flow and brain temperature soar to new heights, but large muscles of the body are completely paralyzed: arms, legs, and trunk cannot move.

The NREM-REM cycle varies from 70 to II0 minutes but averages around 90 min. In the early part of the night sleep is dominated by the NREM state, but as the night progresses, the periods of quiet sleep become shorter and the REM episodes longer. The first REM period lasts 10 min., but by early morning they can last as long as an hour. So we are believed to go into REM sleep and dream roughly every 90 min. all night long. So most of us sleep in two distinct ways: REM sleep, when we dream, and "quiet sleep" when we simply sleep.

EXERCISES

1. How long do most of people spend their lives asleep?

2. Why has the nature of sleep puzzled mankind for thousands of years?

3. Sleep is remarkably passive one, isn't it?

4. Do the brainwaves become larger or smaller as the sleep becomes deeper?

5. How many kinds of sleep does human being experience every day?

6. Who firstly became interested in the slow rolling eye movements?

7. What term did Dr. Kleitman coin for rapid-eye-movement sleep?

8. What is the other name of "quiet sleep"? Why is it called so?

9. What usually happens in human being's organism during REM sleep?

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