Everyone "knows" intuitively what sleep is. But what is sleep really? What are the anatomic, biochemical, and neurophysiologic parameters that define sleep and sleep disorders? Major strides forward have been made in recent years to help clarify some of the mysteries of normal and abnormal sleep.

Sleep is a period of rest and rejuvenation for body and mind. It is a normal, recurring physiologic state from which one can be readily aroused. There are two stages of sleep: light sleep and deep sleep. Light sleep is also referred to as slow-wave, non-rapid eye movement (nonREM), and synchronized sleep. Deep sleep is also known as fast-wave, rapid eye movement (REM), desynchronized, paradoxical, and dreaming sleep.

The amount of sleep required varies from person to person. Infants require much more sleep than do adults. Infants have a higher proportion of REM sleep to light sleep. Of the 18 hours per day that infants sleep, about half is REM sleep. Adults, on the other hand, average around seven hours of sleep per day--less than two hours of which is REM sleep. Insomnia is very common in the elderly. The amount of rapid eye movement (during REM sleep) decreases with age. Women usually have a higher REM density than men have.

There is a surprising number of sleep disorders. They can be caused by internal problems as with depression, epilepsy, Alzheimer's disease, and narcolepsy. They may be instigated by external factors as noise, drugs, alcohol, and shift work. About 25% of the population suffers from sleep disturbances. Women appear to be at a higher risk for sleep disturbances than men. Sleep disorders constitute an important health problem.

Light sleep is a dreamless phase in which EEG (electroencephalogram) recorded brain waves are of low frequency but of high voltage. About 80% of sleep in the adult is light, nonREM sleep. Deep, REM sleep is characterized by lower voltage and higher frequency EEG waves. REM sleep occurs 2-5 times per night with each incidence lasting from a few minutes to over an hour.

The sleep cycle is a continuous and dynamic process. Sleep and wakefulness are subserved by antagonistic brain activities. Sleep-wake states require the coordinated output of several functionally diverse neuron pools. REM sleep and nonREM sleep are different states modulated by the thalamus, hypothalamus, brain stem nuclei, and the basal forebrain. The thalamus is known to play a major role in the regulation of the wake-sleep rhythm. The activity of billions of thalamic and cortical neurons must be synchronized during the different phases of sleep.

A number of neurotransmitters and hormones interact throughout the sleep cycle. The level of wakefulness is modulated by acetylcholine, serotonin, histamine, and norepinephrine. Growth hormone-releasing hormone has an effect on nonREM sleep, while prolactin may help stimulate REM sleep. The central circadian clock of the mammalian suprachiasmatic nucleus influences the timing and duration of sleep. The serotonin S2C receptor is a key actor in human slow-wave, light sleep.

REM sleep is characteristic of most, if not all, terrestrial placental mammals and marsupials. The function(s) of REM sleep is under intense investigation. It appears to play a role in information processing and memory. REM sleep is predominately mediated through acetylcholine-containing neurons (peribrachial regions) and associated cholinergic mechanisms. Serotonergic (dorsal raphe) and noradrenergic (locus coeruleus) neurons act to inhibit REM sleep.

REM sleep and possibly dreaming may be involved with the integration of information. Memory consolidation in humans is postulated to take place during REM sleep. Information acquired during waking hours is reactivated in hippocampal structures during REM sleep which may facilitate memory retention.

• basal forebrain: Lower area of the front part of the brain best known for its role in certain kinds of learning. But projections from the basal forebrain also have a larger role in cortical processing of information.
• brain stem nuclei: The part of the brain on which the cerebral hemispheres rest; in general, it regulates reflex activities that are critical for survival (e.g., heart rate and respiration).
• central circadian clock: The internal 24-hour rhythm which governs the behaviour of many organisms.
• EEG: Electroencephalography, the recording of brain waves by means of electrodes attached to the skull.
• hippocampal structures: Area of the limbic system important for memory.
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• hormones: Chemicals used by endocrine system to transmit messages.
• hypothalamus: Brain structure that monitors internal environment and attempts to maintain balance of these systems. Controls the pituitary.
• insomnia: Difficulty in sleeping. A confusing term-though ubiquitously employed because it is used to indicate any and all gradations and types of sleep loss.
• narcolepsy: A disorder of sleep associated with excessive daytime sleepiness, involuntary daytime sleep episodes, disturbed nocturnal sleep, and cataplexy.
• neurotransmitters: A chemical released by a neuron at a synapse to relay information to an adjacent cell.
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• sleep disorders: Various disorders of sleep: primary sleep disorders such as insomnia and secondary sleep disorders due to medical conditions.
• suprachiasmatic nucleus: A discreet brain region lying within the hypothalamus and responsible for the generation of circadian oscillations.
• thalamus: The key relay station for information from the sensory pathways to the cerebral cortex.

Editor

Frederic A. Spangler, Ph.D.

• CSA Senior Science and Internet Editor; Manager, Web Resources Group

• B.S. (Biology; minor concentrations in Mathematics, Biophysics, and Earth Sciences), Pennsylvania State University

• Ph.D. (Physiology and Biophysics; thesis research area: Biochemical Endocrinology; minor fields of study: Neurobiology, Pharmacology), The Georgetown University Medical Center

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