The two major phases of sleep are rapid eye movement (REM) sleep and nonrapid eye movement (NREM) sleep (Gillin et al., 2000). REM sleep is often called dreaming (or D) sleep because dreams are reported by about 70-80% of persons awakened during this period. It also has been referred to as paradoxical sleep because the brain paradoxically seems to be in an activated state that is similar to, but not identical to, the waking state. For example, brain metabolism is normal or slightly increased during this period. REM sleep is characterized by an activated electroencephalogram (EEG) pattern (low-voltage, fast-frequency brain waves), muscular paralysis (with the exception of diaphragmatic and ocular muscles), periodic bursts of rapid eye movements, and instability of the autonomic nervous system (e.g., variable blood pressure, heart rate, and respiration). In men, penile erections occur during REM sleep, which can be evaluated in sleep studies to distinguish organic and psychological causes of impotence. Finally, nightmares occur almost exclusively during REM sleep. By contrast, NREM sleep encompasses the other four stages of sleep, including the deepest state of sleep, where the EEG pattern is less activated (high-voltage, slow-frequency brain waves), autonomic function is slower and steadier, brain metabolism is decreased, and there is no muscular paralysis or rapid eye movements. Moreover, less than 30% of persons report dreaming during NREM sleep. For this reason, night terrors (which differ from nightmares) occur during NREM sleep.
The time from falling sleep to the first onset of REM sleep is referred to as REM latency, which is normally about 70-100 minutes and progressively shortens in normal elderly persons to about 55-70 minutes. The amount of REM sleep also tends to decline with age. Approximately 50% of the sleep in babies is spent in REM sleep, which decreases to about 25% by age 4, remains relatively constant throughout adult life, and then begins to decline again after age 60. The number of rapid eye movements during a REM period is referred to as REM density. REM and NREM sleep oscillate throughout the night with a cycle length of approximately 90-100 minutes. The relative proportion of time spent in REM sleep increases during the course of the night while NREM time decreases. Sleep deprivation is normally followed by several nights of increased REM sleep with shortened REM latency, referred to as REM rebound.
sleep and dreaming have long been an area of clinical and scientific interest in psychology, psychiatry, and neuroscience (Nofzinger et al., 1999). For example, the similarity between hallucinations and the often bizarre and strange content of most dreams naturally stimulated interest in studying REM sleep in patients with schizophrenia and other psychiatric disorders. Although such studies did not support the hypothesis that hallucinations represent waking dreams, further research in depression often found that REM latency is shortened to less than 60 minutes, REM density and the amount of REM sleep are increased, and the distribution of REM sleep is shifted to the earlier part of the night, compared to normal subjects. These findings had been considered a potential biological marker for depression, but shortened REM latency also has been found in some patients with other psychiatric disorders. Patients with schizophrenia and psychotic depression can have extremely short REM latencies, sometimes occurring immediately at the onset of sleep (sleep-onset REM) (Howland 1997). Increased REM sleep and shortened REM latency also occur in patients during the acute period of abrupt withdrawal from alcohol, benzodiazepines, and other sedative-hypnotic drugs. There is some evidence that shortened REM latency might be a genetic marker for depression within families. Many effective treatments for depression (including antidepress-ant drugs and electroconvulsive therapy) are associated with a decrease (or even suppression) of REM sleep and a lengthening of REM latency. Curiously, total sleep deprivation and even more selective sleep deprivation (i.e., waking patients during the onset of REM sleep throughout the night) has an antidepressant effect in many depressed patients, but this phenomenon is transient, and depression usually returns again after a complete night of sleep.
Abnormalities in REM sleep have been described in two neurologic disorders. In narcolepsy, sleep-onset REM periods are very common. These patients abruptly fall asleep, and they often report hypnogogic hallucinations, which are vivid dream-like states that are likely related to the rapid occurrence of sleep-onset REM periods. Patients with narcolepsy also develop cataplexy, which is the sudden brief loss of muscle tone during waking periods. This may be related to the muscular paralysis normally associated with REM sleep. These findings suggest that narcolepsy is characterized by dysregulated control of REM-NREM sleep. In REM behavior disorder, the normal paralysis of REM sleep is lost. These patients show complex vocal and motor behaviors during REM sleep and often appear to enact the dream content. This is similar to what is seen in animals with selective brain-stem lesions described previously.
Despite many decades of research, the precise function of sleep is not certain (Reiser, 2001). Sleep studies in various animal species suggest that NREM sleep evolved earlier than did REM sleep. These findings and other studies have suggested that NREM sleep might have a primary
role in the conservation and restoration of energy, whereas REM sleep might be especially important to the development and maintenance of cognitive functioning.
Gillin, J. C., Seifritz, E., Zoltoski, R. K., & Salin-Pascual, R. J. (2000). Basic science of sleep. In B. J. Sadock & H. I. Kaplan (Eds.), Comprehensive textbook of psychiatry (7th ed., pp. 199209). Baltimore: Williams & Wilkins. Howland, R. H. (1997). Sleep-onset rapid eye movement periods in neuropsychiatric disorders: Implications for the pathophysiol-ogy of psychosis. Journal of Nervous and Mental Disease, 185, 730-738.
Nofzinger, E. A., Keshavan, M., Buysse, D. J., Moore, R. Y., Kupfer, D. J., & Reynolds, C. F. (1999). The neurobiology of sleep in relation to mental illness. In D. S. Charney, E. J. Nestler, & B. S. Bunney (Eds.), Neurobiology of mental illness (pp. 915-929). New York: Oxford University Press. Reiser, M. F. (2001). The dream in contemporary psychiatry. American Journal of Psychiatry, 158, 145-153.
Robert H. Howland
University of Pittsburgh School of Medicine
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