By Mindy Weisberger
Learn facts about sleep, during which the body rests, recuperates and performs essential repairs.
Sleep Cycles
One rapid eye movement (REM) stage and three non-REM stages that altogether last about 90 to 120 minutes
Benefits
Physical and mental recuperation and memory consolidation
Disorders
Insomnia, narcolepsy, sleep apnea, night terrors, restless leg syndrome
By some estimates, the average person spends about one-third of their lives sleeping (or trying to sleep). Sleep is a resting state for the brain and body that occurs at regular intervals, and getting enough sleep is essential for maintaining good health.
When a person is asleep, their core body temperature drops. Heart rate and breathing also slow down and overall metabolic activity decreases by about 10%. A sleeping body may seem inactive at first glance, with the brain relatively unresponsive to external stimuli. But in reality, a sleeping brain is a busy brain. Sleep enables the brain to perform essential repairs and maintenance tasks, building connections between cells that optimize the brain's performance when awake. Dreaming during sleep is thought to help the brain organize and stabilize memories, as well as process and store information.
Sleep also enables the body to repair and replenish cellular material that is damaged or depleted. Important bodily functions such as the production of hormones, tissue growth and muscle repair, for example, occur mostly while we are asleep, and a system that flushes toxins and waste from the brain is most active in sleep.
A normal sleep cycle includes two basic stages known as rapid-eye movement, or REM sleep, and non-REM sleep. In each sleep cycle, there are three stages of non-REM sleep that move the sleeper from light sleep to deeper rest, finally leading to one stage of REM sleep. During a typical period of sleep a person will experience four to six sleep cycles, with each cycle lasting about 90 to 120 minutes. With each successive cycle, the non-REM stages shorten and a greater proportion of the sleeper's time is spent in REM sleep.
When a person is sleeping, the electrical activity in their brain produces different wave patterns, depending on the stage of their sleep cycle they're in. These brainwaves vary in frequency, with the waves being more rapid when a person is just starting to fall asleep. The waves then fall into a slower frequency as sleep deepens.
The first part of the sleep cycle is a non-REM stage called N1 that lasts from one to seven minutes. The muscles start to relax, and people are easily woken during this stage of light sleep. This stage is followed by non-REM stages N2 and N3, during which the muscles relax even further as breathing rate and body temperature decrease. The N2 stage lasts about 10 to 25 minutes. The N3 stage, known as deep sleep, lasts around 20 to 40 minutes.
The fourth and final stage, REM sleep, is so named because during this sleep stage the eyes move quickly, though the eyelids remain closed. Muscles other than those that move the eyes become temporarily paralyzed. Most of a person's dreaming happens during REM sleep. About 25% of time spent asleep is in REM sleep; this stage lasts about 10 minutes but gets longer with every cycle, eventually lasting up to one hour a cycle.
