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biorhythm Any biological cycle in the body involving periodic changes in body temperature, pulse, and blood pressure circadian rhythm A biorhythm occurring in periods of twenty-four hours diurnal pattern A cycle that repeats each day non-rapid eye movement (NREM) sleep A progressive, four-stage cycle of sleep that comprises about 80 percent of sleep and aids in physical regeneration of the body systems norepinephrine A stimulant released by the reticular activating system during waking hours that keeps the mind alert rapid eye movement (REM) sleep Paradoxical sleep, or sleep that generally follows each non-rapid eye movement sleep cycle and is believed to relate more to psychological well-being and improved memory rest A state of peace, ease, or relative quiet reticular activating system (RAS) A structure in the brain stem that helps regulate wakefulness and that seems to shut down during sleep serotonin A hormone and neurotransmitter from the pons and the forebrain that may enhance sleep sleep An altered state of consciousness or a change in awareness of and reaction to one's environment, with the eyes usually closed and little or no conscious thought sleep-wake cycle The pattern of sleep and wakefulness in periods of twenty-four hours slow-wave sleep A deep sleep in the fourth and final stage of non-rapid eye movement sleep