![]() ![]() ![]() Someone awakened from N3 sleep is extremely groggy and disoriented, Quan said. Typically lasting 20 to 40 minutes, N3 sleep is when the brain becomes less responsive to external stimuli, and as a result, it is most difficult to wake a person up from this stage, Quan said. A person typically spends more time in the N3 stage during the first half of sleep than the second half, but why this happens is not known. N3 sleep is a period of deep sleep that is needed for an individual to feel refreshed for the next day. ("Delta" waves are a type of slow brain wave typically seen during this stage on EEG in a sleep lab.) Non-REM sleep then progresses into its third stage, which is often referred to as "slow wave," "delta" or "deep" sleep. Adults spend about 55 percent of their total sleep time in stage N2 sleep, Quan said. It's also considered a period of light sleep.ĭuring this stage, eye movement stops, heart rate slows, brain waves become slower and muscles relax even further.Īs sleep cycles repeat throughout the night, a person spends more time in stage N2 sleep than in any other sleep stage, according to the National Institutes of Health. Shortly after N1 sleep ends, a person enters this second stage of non-REM sleep, which typically lasts 10 to 25 minutes, Quan told Live Science. When it occurs, this sudden movement may or may not wake a sleeper up.Īdults spend the least amount of time in stage N1 sleep, which represents about 5 percent of their total sleep time, Quan said. This is a sudden, brief muscle jerk that may happen along with a falling sensation when a person is in bed, he said. ![]() It's normal for a person to experience "hypnic jerks," also known as "sleep starts," during N1 sleep, Quan said. N1 sleep is the first stage entered when taking a nap. Body temperature decreases, and brain waves, if observed on an electroencephalogram (EEG) in a sleep lab, would be seen to slow.Ī person can be easily awakened from N1 sleep, and that individual may not think he or she had been asleep, Quan said. During this stage, heart and breathing rates begin to slow, eye movements also slow, and muscles relax. ![]() This is a relatively light form of sleep that lasts about 5 to 10 minutes. In this first stage of non-REM sleep, a person is making the transition from being awake to falling asleep. When a person gets drowsy, he or she is drifting into N1 sleep, Quan said. Live Science asked Quan for a more detailed explanation of what happens in the body and brain during each of these four stages of sleep. As sleep progresses in a series of four to five sleep cycles throughout the night, the time spent in the REM stage gets longer and the time spent in N3 sleep gets shorter, Quan said. This sequence changes somewhat between the first and second half of sleep. Those were combined into one stage, now referred to as N3.ĭuring sleep, the brain repeatedly cycles through four distinct stages of REM and non-REM sleep in a specific sequence. Before 2007, non-REM sleep was broken down into four stages, but then sleep medicine specialists decided that there was no physiological reason to distinguish between two of the stages, the old stage 3 and stage 4 sleep, he explained. Non-REM sleep is now considered to consist of three stages, known as N1, N2 and N3, Quan said. ![]()
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