In scientific terms, sleep is a state of altered brain activity. During non-REM sleep, brain activity looks very different than it does when you’re awake, but during the REM portion of your snooze, it actually looks very similar to an awake state. In all stages, though, there’s a lot going on.
“Sleep, in a way, is still a big mystery to us,” says Michelle Drerup, PsyD, Director of Behavioral Sleep Medicine at Cleveland Clinic. “We know that in non-REM sleep and when there’s deep sleep, there’s physical restoration occurring, but in REM sleep, there’s still some debate. It is thought that we’re filing things away from the day and [engaging in] memory consolidation.”
During sleep, there are also distinct physical changes in the body, such as changes in eye movement and muscular tension. Further variations in electrical activity in the brain show when each stage of sleep begins and ends. And speaking of those stages, there are four of them, plus the REM phase, and we cycle through them about four to five times each night if we’re getting a proper amount of shut-eye.
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Sleep stage 1
Our breathing and heartbeat become regular, our muscles relax, and our body temperature falls. We become less aware of external stimuli, and our consciousness starts to withdraw from reality. The slightest noise is enough to wake you from this stage, and if you do indeed wake up, you might think you haven’t been asleep at all. You have likely experienced the sensation of falling suddenly, typical of this stage, and some people also experience twitching. Sleep stage 1 can last from five to 10 minutes, according to Cleveland Clinic.
Sleep stage 2
Sleep becomes deeper during this stage, and our muscles relax further. Physical sensations are dampened significantly, and our eyes do not move. Electrical activity in the brain occurs at a lower frequency than when we’re awake. About half of our total sleeping time is spent in stage 2, which means that we’re spending more of our repeated sleep cycles in this stage than in any of the others, according to the National Institute of Neurological Disorders and Stroke. Stages 1 and 2 are known as the light-sleep phase. Together, they last for about 20 to 30 minutes per sleep cycle. We return to stage 2 several times during the night.
If you’re going to take a power nap, it is generally recommended that you wake up during this second stage of sleep. “A 15- or 20-minute nap tends to be really refreshing for people because your brain activity is not into the slow-wave activity of stage 3,” says Drerup. “When you go into that type of sleep, it’s much more difficult to wake, you’re groggy, and you have sleep inertia.”
Sleep stages 3 and 4
We reach the first of our deep-sleep stages, stage 3, after approximately 20 to 30 minutes, and the second, stage 4, after about 45 minutes. At that point, the body is completely relaxed, and we are more or less completely disconnected from reality. If you want to wake someone from deep sleep, you need to make a lot of noise or shake them quite hard. Waking someone from stage 4 is almost impossible—a bit like trying to wake a hibernating bear in the middle of winter. This is the most restful part of the night’s sleep. Muscular activity decreases even further, and our eyes do not move. Stages 3 and 4 make up about 20 percent of our time asleep, but this proportion decreases as we get older. Here’s how you can combat America’s sleep-deprivation crisis—and get more shut-eye.
Between 80 and 100 minutes after falling asleep, the deep-sleep phase ends—something that is often accompanied by a change in sleeping position. Our sleep switches to stage 2 for a few minutes again before the brain abruptly switches to REM sleep, the dreaming sleep stage. Our heart rate increases, our breathing gets quicker, and the electrical activity in the brain creates small, rapid movements in the EEG, similar to those seen when falling asleep. Our muscles are completely relaxed, but our eyes make quick, darting movements while remaining closed. This is where the phrase rapid eye movement (REM) comes from.
Men occasionally experience erections during this phase, and women may have increased blood flow to the genitals. The production of digestive juices also increases. It is during REM sleep that we have most of our dreams. For adults, REM sleep makes up about 20 percent of a night’s sleep. The percentage is considerably higher for infants and small children.
When healthy people are in a state of REM sleep, the muscles of our body are deeply relaxed. If it wasn’t for this, we might act out our dreams, with potentially disastrous consequences. This is most likely the reason the brain puts the body in this deep state of relaxation bordering on paralysis (known as atonia) during this stage. However, when a person is suffering from some conditions, such as Parkinson’s disease, REM atonia does not occur and people can act out their dreams.
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The sequence of sleep stages
Sleep follows a specific sequence of these different stages. We complete a sleep cycle and begin a new one approximately every 80 to 110 minutes, usually around 90 minutes. A night’s sleep begins with a light-sleep phase of varying duration, followed by the first deep-sleep phase of the night and a short REM phase. In the second half of the night, we spend a relatively shorter amount of time in deep-sleep phases while our REM phases tend to be longer. The final REM phase of the night can last for as long as 30 minutes or more. And then, we wake up.
The pattern of REM sleep changes as we grow older. During the first year of life, babies spend most of their time asleep in REM sleep. After the age of four, the proportion of REM sleep falls to about 20 percent of the night. People over the age of 60 spend only about 15 percent of the night in REM sleep. With the exception of infants, people spend most of the night in the light sleep phase.
That said, REM is an incredibly important part of our sleep—and most of us aren’t getting enough of it in our sleep-deprived society. “If we’re trying to get away with sleeping less, we’re getting less REM sleep,” says Drerup, since REM sleep can get short by an alarm clock and a mad dash out of the house in the morning. While there is a general guideline that you should get seven to eight hours of sleep each night, Drerup says that opposed to getting the “right number,” people should focus on improving their quality of sleep first. That includes limiting environmental disruptions, identifying and treating sleep disorders, limiting blue-light exposure to phones and other devices at night, and making sure you’re getting enough activity during the day.
The purpose of sleep stages
Each sleep phase serves a specific physiological purpose. The primary function of both our light-sleep and deep-sleep phases is to have a regenerative effect on various processes in the body.
“We tend to think about sleep as a thing that occurs just at night, but it has consequences on your whole 24 hours,” says Drerup. “Things like your mood and your ability to focus and concentrate have been known for a long time, but now we have more evidence and research tying the lack of sleep to being associated with decreased immune functioning, Alzheimer’s and cardiovascular risk. So less sleep not only affects you that day but has long-term health consequences.”
During non-REM sleep, the body repairs itself, builds bone and muscle, and bolsters the immune system. As you get older, this non-REM sleep decreases significantly—from around two hours per night when you’re under 30 to possibly just 30 minutes when you’re over 65. And during both deep sleep and REM sleep, the brain works to process the impressions and memories of the day. For example, if you get an adequate amount of sleep the night before an exam, including several deep-sleep and REM phases, you’ll be better able to recall the material you’ve studied.
A review of research published in the journal Biology of Mood & Anxiety Disorders in 2015 also indicates that getting a sufficient amount of REM sleep prior to a traumatic or fear-inducing experience, as well as in the early stages of a trauma’s aftermath, may make a person less likely to develop PTSD. The bottom line: Sleep is incredibly important for a variety of reasons, some of which we’re still learning about. That’s why you won’t want to miss these easy ways to sleep better.