Whether one is awake or asleep, the brain emits electrical waves. These waves occur in predictable sleep cycles that can be measured with an electroencephalograph, known more commonly as an EEG. During wakefullness, the waves are recorded at about ten small waves per second., but with the onset of sleep, the waves become larger and slower. The largest, slowest waves occur during the first three hours of sleep when mental activity slows down but does not stop. In fact, if awakened from slow-waved sleep, a person can often remember vague thoughts that occurred during that period of sleep, but the sleeper does not generally dream. Referred to as NREM or non-REM sleep, it is characterized by large, slow waves.

    During sleep, intervals of small, fast waves also occur in patterns similar to those experienced while awake. The eyes move rapidly, and it appears to the observer that the sleeper is watching some event. Sleepers who are awakened during this rapid- eye movement sleep will often recall the details of dreams they have been having. Sleep of this kind is called dreaming sleep or rapid- eye-movement sleep, also known REM sleep. REM sleep is emotionally charged. The heart beats irregularly, and blood pressure may be elevated. In contrast, the body is so still that the dreamer may appear to be paralyzed.

    In a period of eight hours , most sleepers experience from three to five instances of REM sleep. Each instance lasts from five to thirty minutes with an interval of at least ninety minutes between each one. Later instance of REM sleep are usually of longer duration than are instances earlier in the eight-hour period.

    Since people who suffer sleep deprivation experience fatigue, irritably, and loss concentration, we must conclude that sleep is essential because in some way it regenerates the brain and the nervous system. NREM sleep increases after physical exertion, whereas REM sleep tends to increase after a stressful day. Studies suggest that non-REM sleep may be especially helpful in restoring muscle control, whereas REM sleep may be more important in revitalizing mental activity. it appears that both kind of sleep are necessary, and the recuperation of sleep of one kind will not compensate for a lack of the other kind of sleep.

(taken from BARRON'S Practice Excercises for The TOEFL 6th Edition, 2007)

What is the main idea of the second paragraph?