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Sleep may be down time for the sleeper, but it's the graveyard shift for the brain, which works through the night to repair worn organs, build muscle and bone, and process the day's flow of information. Sleep researchers don't completely understand why we sleep, but it may be to allow for this physical and psychological restoration.
During sleep, the brain shifts between two distinct states:
1. The first,
rapid eye movement (REM) sleep, is when the eyes move and their muscles twitch, and the brain has short, rapid electrical wave patterns similar to when we're awake. In this sleep phase, the brain clears short-term memory, and can discharge strong emotions through dreams. With the exception of the muscles controlling the eyes and the diaphragm, skeletal muscle tone is absent during this phase, effectively paralyzing a person while her eyes dart rapidly about beneath the eyelids. Many have experienced a moment early in the sleep cycle in which they are conscious of their semi-awake state but cannot move their bodies. Some researchers have theorized that this state of paralysis occurs to keep the body from acting out the dreams that occur in REM sleep.
2. Non-rapid eye movement (NREM) sleep is a deeper sleep, meaning a person is less likely to be awakened by external stimuli. In a normal night, there are four to five progressively deepening NREM periods, each about 90 minutes long and followed by a period of REM sleep. Seventy percent of human growth hormones are secreted during this NREM sleep. These hormones, stored in the pituitary gland at the base of the brain, help build muscle mass, increase bone density, and aid in the protein synthesis process of all body organs.
During both these phases of sleep, one of the brains principle functions is to process the enormous quantity of sensory information acquired while awake, the majority of which will not make it into long term memory. When, due to sleep deprivation, the brain is not allowed to perform its normal restorative function, the result can be a slowed physical reaction time, lack of creativity, poor memory, and an inability to analyze problems or make decisions.
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