Sleep

Very specific rhythms occur in different stages of the sleep-wake cycle. Beta rhythms are fast, low voltage waves (usually above 15 Hz and below 10 uV) which appear in alert, wakeful states. In the quiet, restful wakeful state prior to sleep onset, or in relaxed meditative state with the eyes closed, the brain displays alpha rhythms of about 8-11 Hz and 50 uV. Fairly high chin muscle activity and slow, rolling eye movements are recorded. Alpha waves disappear with visual imagery or opening the eyes, which causes alpha blocking.

Non-REM sleep is generally believed to occur in four stages and is characterized by lack of dreaming. As the sleeper enters the drowsy, light sleep of stage 1, theta rhythms, ranging between 3.5-7.5 Hz with a lower voltage, appear. The sleeper is generally nonresponsive during this stage, which takes up about 5% of the sleep cycle, but is easily awakened. Once again, high chin muscle activity occurs and there is occasional slow, rolling eye movement.

Within a few minutes, the sleeper enters stage 2 sleep. Brain waves slow even further and spindles (short bursts of electrical impulses at about 12-14 Hz which increase and decrease in amplitude) appear, along with K-complexes (sharp, high voltage wave groups, often followed by spindles). These phenomenon may be initiated by internal or external stimuli or by some as yet unknown source deep within the brain. A few delta waves may appear here. This portion of sleep occupies about 45% of the sleep cycle.

Normally, stage 3 sleep, comprised of 20-50% low frequency/high voltage delta waves, follows stage 2 as a short (about 7% of total sleep) transition to stage 4 sleep, which shows slower frequency higher voltage delta wave activity above 50%. There is virtually no eye movement during stages 2, 3, and 4.

In stage 4 sleep, some sleep spindles may occur, but are difficult to record. This stage occupies about 13% of the sleep cycle, seems to be affected more than any other stage by the length of prior wakefulness, and reflects the most cerebral "shutdown." Accordingly, some researchers believe this stage to be the most necessary for brain tissue restoration. Usually grouped together, stages 3 and 4 are called delta, or slow wave sleep (SWS), and is normally followed by REM sleep.

The sleep cycle from stage 1 through REM occurs three to five times a night in a normal young adult. Stages 3 and 4 decrease with each cycle, while stage 2 and REM sleep occupy most of the last half of the night's sleep. Time spent in each stage varies with age, and age particularly influences the amount time spent in SWS. From infancy to young adult, SWS occupies about 20-25% of total sleep time and perhaps as little as 5% by the age of 60. This loss of time is made up in stage 1 sleep and wakeful periods.

The period comprised of the four stages between sleep onset and REM is known as REM latency. REM onset is indicated by a drop in amplitude and rise in frequency of brain waves. The subject's eyes flicker quickly under the eyelids, dream activity is high, and the body seems to become paralyzed because of the decrease in skeletal muscle tone. After REM, the subject usually returns to stage 2 sleep, sometimes after waking slightly. REM sleep occurs regularly during the night. The larger the brain, the longer the period between REM episodes-about 90 minutes for humans and 12 minutes in rats.

REM sleep is triggered by neural functions deep within the brain, which releases one type of neurotransmitter (chemical agent) to turn REM sleep on and another to turn it off. Whereas autonomic activity (such as breathing and heart rate) slows and becomes more regular during non-REM sleep, it becomes highly irregular during REM sleep. Changes in blood pressure, heart rate, and breathing regularity take place, there is virtually no regulation of body temperature, and clitoral and penile erections are often reported. Most deaths, particularly of ill or aged individuals, happen early in the morning when body temperature is at its lowest and the likelihood of REM sleep is highest.

REM activity is seen in the fetus as early as six months after conception. By the time of birth, the fetus will spend 90% of its sleep time in REM but only about half that after birth. REM constitutes about 20-30% of a normal young adult's sleep, decreasing with age. These observations support one of several theories about our need for REM sleep which suggests that, to function properly, the central nervous system requires considerable stimulation, particularly during development. Because it receives no environmental stimulation during the long hours of sleep, it is possible that the high amount of brain wave activity in REM sleep provides the necessary stimulation.