All species engage in periods (cycles) of sleeping and wakefulness. In part, these cycles are regulated by the nervous system, specifically by a structure in the hypothalamus called the suprachiasmatic nucleus. The reticular activating system, an aggregate of cells in the central part of the brain stem, also affects arousal. Other areas in the medulla and thalamus have also been implicated in the control of sleep and waking. When one is sleeping, responses to environmental stimulation are altered and produce different levels of awareness.

Sleep records. Several types of machines can record bodily changes, including activity of the brain, muscles, and eyes, that occur during sleep.

  • An EEG (electroencephalogram) is a record (made by attaching electrodes to the scalp and face) of patterns of brain activity during periods of sleep and/or waking. EEG records are used to study the sleep process. Records of activity patterns during sleep are different from those obtained during periods of wakefulness. The sleep wave patterns have been characterized by their frequencies, cycles per second:

  • beta waves, 14 to 30 cycles per second

  • alpha waves, 8 to 13 cycles per second

  • theta waves, 4 to 7 cycles per second

  • delta waves, under 4 cycles per second

Brain waves also differ in amplitude (wave height). As one falls asleep, brain waves become slower in frequency and higher in amplitude.

  • An EMG (electromyogram) is a record of muscular tension or relaxation. The device uses electrodes attached beneath the chin.

  • An EOG (electro‐oculogram) is a record of eye movement and is made by means of electrodes attached near the outer corners of the eyes.

All three devices may be used in sleep research laboratories.

Stages of sleep. There are four stages of sleep plus a condition called REM (rapid eye movement) sleep.

  • Stage one sleep is a relaxed wakefulness, with EEG activity of 8 to 13 cycles per second called alpha waves. Eye movement occurs as well as muscular tension.

  • Stage two sleep is marked by bursts of activity called sleep spindles, which occur at approximately 15‐second intervals. Muscle tension is reduced; no eye movement occurs. This stage lasts approximately 20 minutes.

  • Stage three sleep lasts for about 30 minutes; some delta waves occur. Temperature decreases and pulse and breathing rate slow.

  • Stage four sleep is the deepest and consists almost exclusively of delta waves. Eyes do not move.

During sleep, a person moves through sleep stages one to four; the sleep cycle then reverses back through stages three and two to stage one (which, in the reverse order, is called emergent stage one). This cycle generally repeats several times during a night's sleep. As a sleeper passes on through stage two and back into stage one, the eyes begin to dart back and forth; REM (rapid eye movement) sleep and dreams occur. (REM sleep is also called paradoxical sleep because the brain activity, blood pressure, heart rate, etc., closely resemble those of waking consciousness.) Because neither dreams nor rapid eye movements occur in any of the other stages of sleep, those stages are called NREM (non‐rapid eye movement) sleep.

Sleep deprivation. When people go without sleep (sleep deprivation), especially if they have missed REM sleep, drastic changes in their behavior patterns occur. Sleep‐deprived people become tired, irritable, and lethargic.

Dreams. Analysis of dreams, which occur during REM sleep, is an important part of the psychoanalytic treatment for emotional problems. During the treatment process, patients report what they consciously remember about their dreams, the manifest content. In order to better understand their emotional problems, the analyst then has them use a process called free association to help determine the latent content (unconscious portion) of the dream.

Daydreams. Daydreams, as well as sleep, indicate an altered level of consciousness. They represent a shift in attention from ongoing events. People daydream most frequently at night just before they go to bed, but they may daydream any time, especially when engaged in a monotonous task. Daydreams have value because they provide relief from boredom through continuous and frequently changing stimulation; in addition, they permit the planning and imaginary trying out of solutions to problems.

Sleep disorders. The term sleep disorders refers to a variety of problems people have with the process of sleeping.

  • One of the most serious conditions is sleep apnea, a sometimes fatal disorder in which people momentarily stop breathing. At the least, this disruption of the breathing process causes the person to be sleepy the next day.

  • Narcolepsy, another serious sleep disorder, is a condition in which people suddenly fall asleep, even in the middle of another activity.

  • Insomnia, a common problem, is difficulty in falling asleep.

  • Somnambulism is sleepwalking.

  • Nightmares are bad dreams from which the sleeper may wake up aroused and frightened.