Sleep and Dreaming Part 1 - Part 2

Behaviour - Changes in position can occur during waking and in concert with phase changes of the sleep cycle. Two different mechanisms account for sleep immobility. The first is disfacilitation (during stages 1- 4 of NREM sleep). The second is inhibition (during REM sleep). During dreams, we imagine that we move, but we do not.

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Awake - Waking (in the present context) is the phase during which the body prepares for sleep. All people fall asleep with tense muscles, their eyes moving erratically. Then, normally, as a person becomes sleepier, the body begins to slow down. Muscles begin to relax, and eye movement slows to a roll.

Sleep can be divided into five stages. Although the signals for transition between the five stages of sleep are mysterious, it is important to notice that these stages are discretely independent of one another, each marked by subtle changes in bodily function and each part of a predictable cycle whose intervals are observable. Sleep stages are monitored and examined clinically with polysomnography, which provides data regarding electrical states of the muscle (electromyo-gram - EMG), the brain (electroencephalogram - EEG), and eye movement (electrooculogram - EOG) during sleep.

Stage 1 - This is a stage of drowsiness. Polysomnography shows a 50% reduction in activity between wakefulness and stage 1 sleep. The eyes are closed during Stage 1 sleep, but if aroused from it, a person may feel as if he or she has not slept. Stage 1 may last for five to 15 minutes.

Stage 2 - Stage 2 is a period of light sleep during which polysomnographic readings show random fluctuation. These waves indicate spontaneous periods of muscle tone mixed with periods of muscle relaxation. The heart rate slows down, and body temperature becomes lower. At this point, the body prepares to enter deep sleep.

Stage 3 - This stage is known as slow-wave, or delta, sleep. During slow-wave sleep the EMG records slow waves of relatively high amplitude, indicating a pattern of deep sleep and rhythmic continuity.

Stage 4 - This stage is similar to Stage 3 but more intense. The period of non-REM sleep (NREM) is comprised of stages 1 - 4 and lasts from 90 to 120 minutes. In addition, stage 2 and 3 repeat backwards before REM sleep is attained. Therefore, a normal sleep cycle has this pattern: stage 1, 2, 3, 4, 3, 2, REM. Usually, REM sleep occurs 90 minutes after the onset of sleep.

REM - REM sleep is distinguishable from NREM sleep by changes in physiological states, including its characteristic rapid eye movements. However, polysomnograms show wave patterns in REM to be similar to Stage 1 sleep. In REM sleep, heart rate and respiration speed up and become erratic, while the face, fingers, and legs may twitch. Intense dreaming occurs during REM sleep as a result of heightened cerebral activity, but paralysis occurs simultaneously in the major voluntary muscle groups. Because REM is a mixture of encephalic (brain) states of excitement and muscular immobility, it is sometimes called paradoxical sleep. The first period of REM typically lasts 10 minutes, with each recurring REM stage lengthening, and the final one lasting an hour. The percentage of REM sleep is highest during infancy and early childhood, drops off during adolescence and young adulthood, and decreases further in older age.

A sleep cycle comprises five stages of sleep, including their repetition. The first cycle, which ends after the completion of the first REM stage, usually lasts for 100 minutes. Each subsequent cycle lasts longer, as its respective REM stage extends. So a person may complete five cycles in a typical night's sleep.

• Sample tracings of three variables used to distinguish the state are also shown in Figure 10-22: The EMG tracings are highest during waking, intermediate during NREM sleep and lowest during REM sleep. The EEG and EOG are both activated during waking and REM, but inactivated during NREM sleep. Each sample shown is approximately 20 seconds long.

• The three bottom rows in Figure 10-22 describe other subjective and objective state variables such as sensation, perception, thought, and movement.