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The Sleep Cycle
One sleep cycle comprises of four stages and last for about 90-120 minutes. Dreams can occur in any of the four stages of sleep but the most vivid and memorable dreams occur in the last stage of sleep (also commonly referred to as REM sleep). The sleep cycle repeats itself about an average of four to five times per night, but may repeat as many as seven times. Thus, you can see how a person can have several different dreams in one night. Most people, however, only remember dreams that occur closer toward the morning when they are about to get up. But just because you can't remember those dreams does not mean that
they never happened. Some people swear on the fact that they simply do not dream when in reality, they just don't remember their dreams.
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.....Do we dream to forget? Or to remember? The answer seems to lie in new findings about REM sleep and its unique biological function. First, however, let's shatter a myth. Dreams and rapid-eye-movement (REM) sleep are not one and the same. We dream throughout the night, sometimes while in deep sleep--the sleep marked by slow EEG waves, during which the body repairs itself, releasing growth hormone. REM sleep, in contrast, is a violently "awake" sleep; the muscles are at rest but the brain and nervous system are highly active.
The brain cycles through REM sleep about four to six times a night, each time marked by irregular breathing, increased heart rate and brain:! temperature, general physiological arousal, and, in men, erections. Arousal is such that ulcer sufferers secrete twenty times more stomach acid in REM than in non-REM sleep.
The first REM cycle follows ninety minutes of slow-wave deep sleep and lasts about ten minutes. REM cycles lengthen through the night and the dreams in them get more bizarre and detailed, like wacky movies. REM dreams tend to be uniformly more emotional and memorable than non-REM ones. One of the most interesting aspects of REM sleep is that, for its duration, we are paralyzed from the neck down, and our threshold for sensory input is raised, so that external stimuli rarely reach and wake us. The brain is soaked in acetylcholine, which seems to stimulate nerve cells while it strips muscles of tone and tension. At the same time, serotonin levels plummet. The changes are swift and global. It's as if during these cycles we are functioning with a different brain entirely......