Sleep is a physiological condition of partial loss of consciousness.. Generally, a sleeper --as distinct from a person in a coma-- can be aroused by varying levels of stimulation. Sleep physiologists describe two main categories of sleep: 1. non-rapid eye-movement sleep (NREM ). 2. rapid eye-movement sleep (REM) The amount of sleep needed by a healthy person varies with age; 1. In general,the amount of sleep needed decreases with age, from infancy to adulthood. 2. At a certain stage of adulthood the necessary amount of sleep plateaus . 3. with the arrival of old age sleep need again starts to decline . Not only the amount of sleep, but the kind of sleep necessary (REM vs NREM) varies with age It is known that infants spend 50% of their sleep time in REM sleep. By age 10 school age children seem to require only 25% of their sleep to be REM sleep. This decline in time spent in REM sleep declines throughout life.
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Physiologists have two large categories of :sleep NREM sleep is divided into four stages based on type and frequency of EEG wave representation of brain activity and on the level of arousal , eye movements NREM sleep has four stages : The diagnostic wave forms are alpha waves and delta waves. . Alpha waves predominate in NREM sleep stages but the wave form changes to delta waves as the sleeper drifts into REM sleep. During REM sleep the eyes move rapidly under the eyelids . This is the dreaming stage of sleep. REM sleep and NREM stages alternate throughout a sleep session or episode The need for REM sleep varies with age. It is important that people get enough REM sleep for adequate REM sleep is necessary for emotional stability. The amount of REM sleep needed by an individual varies with . The following is a commonly used government estimation: An infant needs about 50% of his/her sleep to be REM sleep. By age 10, that requirement drops to 25%.. Between 16 and 25 years of age that necessary percentage is only 20%. The necessary amount continues to decline, and by age 50 the adult spends only 3.5% of his sleep session in REM sleep. As aging progresses the amount of REM sleep needed gets less and less by several minutes per decade of increased age.