There are four stages in each sleep cycle, commonly divided in two groups: REM (rapid eye movement) and NREM (non-rapid eye movement) sleep. They are so different from each other that physiologists identify them as distinct behavioral states. In NREM stages, our brain uses less energy and has a decreased activity. REM, is also known as paradoxical sleep because our brain acts similarly to when we’re awake.
The sleep cycle moves from light sleep (stage 1), to deep sleep (stage 3) and goes back to stage 3, then stage 2 and finally REM sleep (stage 4). A complete sleep cycle takes between 90 to 110 minutes, occuring 4-6 times in a good night’s sleep.
Stage 1 - NREM
It’s the first stage that takes place after you decide to sleep. It lasts between 1 and 10 minutes. It’s a very light sleep, you can quickly return to be fully awake. Even though you are asleep, you may wake up feeling you didn’t sleep at all. Your muscles are not inhibited yet. You breathing pace decreases, along with body temperature, blood pressure and heartbeats.
Hypnic jerks also happens in this stage. It’s the rapid waking up, sometimes along with the sensation of falling down. Some scientists believe it’s a reflex humans developed during the evolutionary process to avoid falling off trees during their sleep.
Stage 2 - NREM
We spend around 45% of our sleep in this stage. It becomes harder to wake up. Blood pressure and other metabolic functions slow down even more. It lasts around 20 minutes and body temperature decreases to enter into deep sleep.
The first two stages are called light sleep. While resting usually occurs during the latter stages, there are some evidences that light-sleep is important to boost our ability to learn.
Stage 3 - NREM
Also called ‘slow-wave sleep’ or ‘Delta sleep’, this stage starts around 45 minutes after falling asleep. Our brain waves get slower and bigger. At this point, it’s very difficult to wake you up. You may show no reaction to noises and movements. Waking up during this stage makes you feel disoriented for a few minutes. This stage is more known as ‘deep sleep’. It’s during this stage where one might experience sleepwalking, night terrors, talking during sleep and bedwetting. Those behaviors are known as parasomnias an usually happen during the transition between non-REM and REM sleep.
It’s during slow-wave sleep where most of the restaurative functions occur. The decreased metabolic rates, oxygen consumption and heart rate allows the cells to recover from damages caused by oxidative processes.
It’s also when explicit memory is processed. This includes episodic memory, which stores personal experiences, and semantic memory, which stores factual information.
Stage 4 - REM
Known as Rapid Eye Movement because of the intense eye movement during this stage. It’s during this stage where the dreams happens. Even though our eyes are moving rapidly, most of our body is paralysed. Some scientists believe we developed such mechanism to avoid potentially dangerous reactions to dreams.
It starts after having been asleep for at least 90 minutes. The first sleep cycle each night has a short REM stage and its length increases in the following cycles.
This sleep stage wasn’t known before 1954, when new equipments to monitor brain activity were developed. Scientists previously believed that most brain activity ceased during sleep. To this day, the functions of REM sleep are not well understood. Experiments showed that prolonged REM sleep deprivation leads to several behavioral and physiological abnormalities in humans and animals.
REM sleep seems to be related to procedural memory, which stores new techniques of problem solving and skill acquisition as well as the new ways we learn to move our body.
Sleep is still not well understood by science. We only have some insights on how its different stages interfere with our brain. Being aware of how sleep works is a way to better understand yourself, improve sleep and our overall quality of life.