Sleep is a universal need of all higher life forms including humans, absence of which has serious physiological consequences, yet a lot of people know little about sleep. Its function remains to be fully elucidated. Humans spend about one-third of their lives asleep.
Sleep is a naturally recurring state of mind characterized by altered consciousness, relatively inhibited sensory activity, inhibition of nearly all voluntary muscles, and reduced interactions with surroundings. It is distinguished from wakefulness by a decreased ability to react to stimuli, but is more easily reversed than the state of hibernation or of being comatose. Mammalian sleep occurs in repeating periods, in which the body alternates between two highly distinct modes
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Which include an increasing proportion of paradoxical (REM) sleep as they repeat. This rhythm is called the { ultradian sleep cycle} . Sleep proceeds in cycles of REM and NREM, usually four or five of them per night. A sleep episode begins with a short period of NREM stage 1 progressing through stage 2, followed by stages 3 and 4 and finally to REM. However, individuals do not remain in REM sleep the remainder of the night but, rather, cycle between stages of NREM and REM throughout the night .NREM sleep constitutes about 75 to 80 percent of total time spent in sleep, and REM sleep constitutes the remaining 20 to 25 percent. The average length of the first NREM-REM sleep cycle is 70 to 100 minutes. The second, and later, cycles are longer lasting—approximately 90 to 120 minutes .In normal adults, REM sleep increases as the night progresses and is longest in the last one-third of the sleep episode. As the sleep episode progresses, stage 2 begins to account for the majority of NREM sleep, and stages 3 and 4 may sometimes altogether …show more content…
REM sleep is turned on by acetylcholine secretion and is inhibited by neurons that secrete monoamines including serotonin. This level is also referred to as paradoxical sleep because the sleeper, although exhibiting high-frequency EEG waves similar to a waking state, is harder to arouse than at any other sleep stage. Vital signs indicate arousal and oxygen consumption by the brain is higher than when the sleeper is awake. An adult reaches REM approximately every 90 minutes, and remains in REM sleep for longer during latter half of sleep. REM sleep occurs as a person returns to stage 1 from a deep sleep.
The function of REM sleep is uncertain but a lack of it impairs the ability to learn complex tasks. One approach to understanding the role of sleep is to study the deprivation of it. Functional paralysis from muscular atonia in REM may be necessary to protect organisms from self-damage through physically acting out scenes from the often-vivid dreams that occur during this stage. Infants spend almost 50% of their time in REM sleep. Adults spend nearly half of sleep time in stage 2, about 20% in REM and the other 30% is divided between the other three stages. Older adults spend progressively less time in REM