While you rest the brain is working hard to store memories, heal tissues, and restore the body. To do all this it passes through a total of four stages of sleep several times each night. These stages are categorised as both REM and Non-REM sleep. Before 2007 experts considered there to be 5 sleep stages, including REM, but most medical professionals today would agree that there are only 4.
Source: “Sleep and Sleep Stages” – Lumen Learning
A full sleep cycle lasts around 90 minutes and consists of all 4 stages. Sleepers move from stage 1, light sleep and through all the phases until they are in stage 3, deep sleep. The last is where dreaming occurs and is called Rapid Eye Movement (REM) Sleep.
Sleepers do not simply end a cycle and begin again. After moving from light sleep to deep sleep, the cycle reverses from stage 3 deep sleep to stage 1 light sleep before moving into REM and beginning with stage 1 once again.
For centuries people believed sleep to be a passive event, but in 1953, through the use of Electroencephalography (EEG) machines, scientists Nathaniel Kleitman and Eugene Aserinksy were able to determine that the body passes through several complex stages of sleep. EEG’s record electrical brain activity. With electrodes placed on certain parts of the scalp, this non-invasive method is often used to measure wave patterns in the brain while participants are sleeping. In this setting, the EEG is placed on the sleeper and monitored by scientist and medical professionals during a Polysomnography, better known as a sleep study. This is done to better understand sleep patterns and diagnose potential sleeping disorders.
Light sleep is characterized by stages 1 and 2 of the sleep cycle. During these phases, the sleeper’s subconscious is much more aware of external stimuli, making it easier to wake up. It is believed that sleep spindles in phase 2 may be important to learning, as this may be where memories are processed from short term to long term memory.
Deep sleep, or stage 3, is when the body works to heal tissues and grow new cells. As an individual’s sleep cycle progresses they spend less time in deep sleep and more in REM. When people feel exhausted due to lack of rest, it may be because they are not spending enough time in slow-wave sleep.
Stage 1: Light Sleep
This is when the body is first falling asleep. If aroused in this phase, many people would not even realise they were asleep. Although the eyes are closed, sleepers are typically still aware of their surroundings.
Heart and breathing slow
Slows by about 50%
Stage 2: Intermediate Sleep
Sleepers fall into a deeper sleep and their bodies go into a more relaxed state that would be harder to wake up from. With a full night’s rest, this stage may be where sleepers experience the most sleep.
Body temperature drops, digestion speeds decrease, respiratory and heart rates rate slows down
Continues to decrease and brain waves spread out, there are intermittent sleep spindles, a higher burst and frequency of brain waves. The cause of these spindles is unknown
Stage 3: Deep Sleep
This phase is often referred to as slow-wave or delta wave sleep. There are two parts to this stage, with the second part being more intense. Sleepers are not as affected to external stimuli in this phase, making it much harder to arouse them.
The body repairs tissues, builds muscle and bone, and reinforces the immune system. Blood pressure drops further as do respiratory rate and body temperature
Slower and larger brain waves appear as the body relaxes and begins to repair itself
Stage 4: REM
After reversing back through the other three stages of sleep, the body enters REM or dream sleep. To prevent the sleeper from acting out dreams, the brain releases chemicals that paralyze the body, allowing the muscles to relax. This occurs anywhere between 90 to 120 minutes after falling asleep.
Rapid eye movement, accelerated respiration and heart rate, increased brain activity, and muscle relaxation
Rapid increases in cerebral activity in part due to dreaming