Studies have reported a decline in sleep duration and sleep quality over the years in many modern societies. Older adults tend to have less consolidated sleep with more frequent awakenings. In general, studies have shown that the percentage of NREM sleep increases and the percentage of REM sleep decreases with increasing age. However, no consensus reached regarding sleep architecture or nap-related variables as an indicator of good sleep quality. Sleep continuity variables including 1) sleep latency of 15 minutes and less, 2) one or fewer awakening of more than five minutes per night, 3) wake time after sleep onset of 20 minutes and less, and 4) sleep efficiency of 85% and more are indicators of good sleep quality regardless of age. A systematic review in 2016, involving 277 studies, has identified all possible sleep indicators and came out with the first sleep quality recommendation, which the National Sleep Foundation endorsed. Other than adequate duration, healthy sleep comprises good quality. A generally accepted assumption is that amount of sleep is enough if the individual wakes up feeling well-rested and perform well during the day. However, there is no ideal duration of sleep required per night. In a meta-analysis that involved 5273 healthy adults, total sleep time decreased by approximately 10 minutes for each decade of age. In general, sleep duration decreases with increasing age. As recommended by the National Sleep Foundation, in a healthy individual, the recommended sleep duration for younger adults is seven to nine hours, and for older adults is seven to eight hours. Sufficient sleep is needed to provide recovery after preceding waking activities and ensure optimal functioning during subsequent wakefulness. Healthy sleep is essential for neural development, learning, memory, cardiovascular and metabolic regulation. The cycle is repeated three to six times, and as sleep episode progresses, the duration of stages of NREM sleep shorten while the duration of REM sleep lengthens. NREM sleep is then followed by REM sleep, which is known for the most vivid dreams and bodily movements. It progresses through NREM stage 2, stage 3 and stage 4, the deepest sleep stage. Sleep episode in adults starts with a brief period of NREM stage 1, the lightest sleep stage. NREM sleep is further divided into four stages, which is a continuum of relative depth. Throughout the period of sleep, the body will cycle periodically between non-rapid eye movement (NREM) sleep and rapid-eye-movement (REM) sleep. Sleep is a state of reversible unconsciousness in which the brain is less responsive to external stimuli.
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