Types of Sleep

Psychologists distinguish between two major categories of sleep: dream sleep and non-dream sleep. Subjects who are observed to experience rapid eye movement (REM) during sleep are most likely to report dreaming. Thus while REM and dreaming are not the same thing, REM is generally interpreted as a sign of dreaming, and non-REM (NREM) sleep is generally expected to be non-dream sleep.

Types of Sleep
1. Non-Dream Sleep
Upon first going to sleep, we usually enter non-dream or NREM sleep. There are four levels or stages of NREM sleep: Stage one, stage two, stage three and stage four. Typically, we progress in order through these four stages -- from stage one through stage four -- during the first 30 - 45 minutes of sleep. After that we move between them throughout the night, spending proportionately larger amounts of time in stages two and three.

The EEG pattern that characterizes ordinary wakefulness is the beta rhythm, a low amplitude pattern of "fast" activity of twelve or more cycles per second. With the onset and deepening of sleep, the EEG patterns change from fast rhythms of low amplitude to slow rhythms of high amplitude.

Stage one: This most superficial stage of sleep is not actually sleep at all in the usual sense but simply the first stage of relaxing into sleep. It is characterized by a passive, restful, state of mind. The EEG is dominated by alpha rhythms in the range of eight to twelve Hz.

Stage two: This stage carries a feeling of drifting off to sleep. There may be a pleasant sense of falling, or the presence of hypnogogic images: vivid but usually stationary images that drift before the mind's eye. These are not dreams, which are much more complex than hypnogogic images. The EEG in stage two sleep exhibits slower and larger theta rhythms, ranging from about four to seven Hz. The presence in the EEG record of brief bursts of high amplitude activity called sleep spindles indicate that the person is drifting into the deeper stages of sleep.

Stage three: This is considered an intermediate level of sleep. It is during this stage that most large body movements occur during the night. The EEG pattern seen in this stage is termed delta rhythm. Delta rhythm can range from one to three Hz, but tends towards two or three Hz in stage three.

Stage Four: This is the deepest stage of NREM sleep. It is also the most restful stage, though we typically spend only about 20% of the night in it. In this stage, the body is deeply relaxed and there is little movement. The EEG seen here is delta rhythm and is very large in amplitude and very slow -- close to one Hz.

Periods of stage four sleep may be relatively long early in the sleep cycle but become shorter as the night goes on. Sleepwalking and sleep talking occur in this stage and are not associated with dreaming. Nightmares, different from ordinary bad dreams, can also occur in this stage. Nightmares or "night terrors" are characterized by unpleasant emotional arousal but are not necessarily accompanied by vivid visual or auditory images. Nightmares are not uncommon in children, who usually grow out of them.

2. Dream Sleep
About every 90 minutes during the course of the night the EEG pattern makes a rapid transition from the deeper stages (three and four) to a rhythm that resembles that of stage one or two. This new sleep pattern is associated with dreaming (dream sleep). It has also been called paradoxical sleep because according to the EEG the individual appears only lightly asleep, while in fact he or she is behaviourally harder to awaken than even from stage four, the deepest sleep.

Dream sleep is also termed REM sleep, because of the tendency of the eyes to move quickly under the closed eyelids -- as if the dreamer's eyes were following dream activities. During REM sleep, the large skeletal muscles of the body become completely flaccid, though movement may be seen in the extremities.

Unlike stage four sleep, periods of REM tend to be very short during the first hours of the night, growing in length as the night goes by. They may be as long as 30 - 45 minutes by the early hours of the morning.



There is one altered state of consciousness that we have all experienced -- dreaming sleep. Dreams are so different from our normal state of awareness that they were once seen as supernatural in origin, and sometimes as vehicles for prophecy. In the account of Pharaoh's dream in the book of Genesis, for example, the appearance of seven fat and seven thin cattle is interpreted by Joseph as predicting the advent of seven years of plenty followed by seven years of famine.