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Sleep is a biological imperative critical to the maintenance of mental and physical health. It is a state of lessened consciousness and decreased physical activity during which the organism slows down and repairs itself. The sleep cycle involves two distinct phases that alternate cyclically from light sleep to deep then deeper and deepest sleep throughout the sleep period. There are two main phases of sleep.
The timing and progression of the sleep cycle and the total amount of nightly sleep required for optimal
Sleep begins in stage one of the sleep phase known as NREM, or non-rapid eye movement, sleep. NREM sleep has four stages: light sleep, deeper sleep, and two stages of deepest sleep. Stage one is the "drifting off" period of light sleep in the transition between wakefulness and sleep and comprises about 5 percent of the entire sleep period. Stage two sleep involves a change in brain-wave patterns and increased resistance to arousal and accounts for 45–55 percent of total sleep time. Stages three and four are the deepest levels of sleep and occur only in the first third of the sleep period. NREM stage four sleep usually takes up 12 to 15 percent of total sleep time. Sleep terrors, sleep walking, and bedwetting episodes generally occur within stage four sleep or during partial arousals from this sleep stage.
It typically takes about 90 minutes to cycle through the four deepening stages of NREM sleep before onset of the second phase of sleep known as REM or dream sleep.
Author Info: Clare Hanrahan, Thomson Gale, Gale, Detroit, Gale Encyclopedia of Children's Health, 2006This feature is for informational purposes only and should not be used to replace the care and information received from your healthcare provider. Please consult a healthcare professional with any health concerns you may have.
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