Proper dreaming occurs during sleep, but it is not just a part of ordinary sleep. Sleep alternates several times during the night with periods of very different kinds of sleep.
In normal sleep the brain produces big, slow waves of delta rhythm, the eyes are still and the heartbeat is regular. However many muscles, particularly the throat muscles are quite tense. In REM (Rapid Eye Movement) periods of sleep the brain produces more rapid waves, almost like those when we are awake, the eyes move rapidly back and forth and the heartbeat becomes irregular. Despite this, the muscles become less tense, even the throat, and we are much more difficult to wake.
It usually takes about two hours of normal sleep before we enter REM sleep. In sleep tests, someone who is awoken every two hours, or before REM, will start to show signs of sleep deprivation. As this state increases, the sleeper tends to go into REM sleep straight away, as if to make up the deficit. It seems that both normal and REM sleep are important, but for different reasons.
Sleep is the time when most of the regeneration of cells occurs. In normal sleep it seems that mostly body tissues are affected and when a person has a physically strenuous day, then he or she will usually spend more time in normal sleep.
Little is known about what occurs during REM sleep other than this is the time when significant electrical energy flows to a part of the brain just below the cortex where it is thought that memory is contained.
Dreams tend to start off with a subject or situation relative to the experiences of the previous day before shifting to earlier periods of life. This suggests that our minds are re-running our experiences and comparing them to past experience before filing them away in the memory bank.