In mammals, the overall sleep cycle is comprised of two broad alternating cycles, REM (rapid eye movement), and NREM (non-rapid eye movement), which itself consists of three individual phases. (Note: REM is pronounced like a word ["rehm"], not as a series of initials. NREM, or non-REM, is pronounced phonetically as well, with the "N" standing alone ["en-rehm"].)
In electroencephalography, the REM phase of sleep is easily identified by its paradigmatic fast, small-amplitude waves. (Also typical are the rapid eye movements for which the REM phase is named.) For this reason REM sleep is sometimes also called "active sleep." Many sleep experts think that these eye movements are in some way related to dreams. Sleep proceeds in cycles of REM and NREM, usually four or five per night. These cycles typically last between 90 and 110 minutes.
Stages of NREM Sleep
Stage 1 NREM (N1)
After a person falls asleep, he or she enters into what is known as stage 1 of non-REM sleep. This stage is sometimes referred to as somnolence, or drowsy sleep. Sudden twitches known as hypnic jerks are associated with this stage. The sleeper loses some muscle tone and most consciousness of the external environment. This stage lasts around ten minutes, during which a person can still be woken easily. A sleeper who has experienced only stage 1 NREM sleep may not even realize they slept at all. Stage 1 NREM is characterized by:
- slow, even breathing,
- regular heartbeat;
- lowered brain temperature.
Stage 2 NREM (N2)
Stage 2 non-REM sleep is characterized by sleep spindles and K-Complexes.
A sleep spindle is a burst of oscillatory brain activity visible on an EEG that occurs specifically during stage 2 sleep. Sleep spindles are generated in the reticular nucleus of the thalamus, and may represent periods in which the brain is inhibiting processing to keep the sleeper in a tranquil state. During sleep these spindles are seen in the brain as a burst of activity immediately following muscle twitching. Spindles have been shown to aid sleeping in the presence of disruptive external sounds, and have also been associated with the integration of new information into existing knowledge, as well as with directed remembering and forgetting.
K-complexes are high-voltage events that occur more frequently in the first sleep cycles. K-complexes are thought to have two main purposes:
- to limit brain activity in response to stimuli that are not dangerous, and
- to help with sleep-based memory consolidation.
Sleep spindles and K-complexes
Sleep spindles and K-complexes are defining characteristics and indicate the onset of stage 2 NREM sleep.
During this stage, muscular activity decreases, and conscious awareness of the external environment disappears. This stage occupies 45–55% of total sleep in adults. It is also characterized by:
- high-amplitude brain waves;
- sporadic short periods of increased activity;
- heightened sensitivity to sound stimuli (sleeper may wake up);
- slowed bodily functions (e.g., lower blood pressure; decreased cardiac and metabolic activity).
Stage 3 NREM (N3)
Stage 3 of non-REM sleep is considered the start to "deep sleep." A sleeper first enters stage three approximately 30 to 45 minutes into sleeping. This is the stage in which different forms of parasomnia, such as night terrors, nocturnal enuresis, sleepwalking, and somniloquy (sleep talking) typically occur. This stage, formerly divided into two separate stages (3 and 4), is called slow-wave sleep, or SWS. In SWS, the sleeper is less responsive to the environment; many environmental stimuli no longer produce any reactions.
It is characterized by the following:
- Delta waves, or, very slow brain waves.
- Deeper sleep; it is much harder to wake up someone in this phase of sleep than someone in stage 1 or 2 sleep.
- If awakened during this phase, a person will likely experience disorientation and grogginess.
The sleeper will go through NREM stage 2 once again before entering REM sleep.
REM Sleep
The sleeper now enters rapid eye movement (REM) sleep. In this stage, most muscles are paralyzed. This level is also referred to as paradoxical sleep because the sleeper, although exhibiting EEG waves similar to a waking state, is harder to arouse than at any other sleep stage.
REM sleep accounts for 20–25% of total sleep time in most human adults. The criteria for REM sleep include rapid eye movements as well as a rapid low-voltage EEG. During REM sleep, EEG patterns return to higher-frequency saw-tooth waves. Most memorable dreaming occurs in this stage. Partial paralysis occurs, perhaps to protect organisms from self-damage through physically acting out scenes from the often-vivid dreams that occur during this stage. Vital signs indicate arousal, and oxygen consumption by the brain is higher than when the sleeper is awake.
Sleep Cycles
Sleep has four distinct phases, or stages. Sleep progresses from stage 1 to stage 2 to stage 3, and then back to stage 2 before transitioning into the REM phase. Once the REM phase is over, stage 2 will repeat. A person will complete this entire cycle about four or five times given a full night of sleep.
A typical night's sleep
This sample hypnogram (EEG of sleep) shows how the proportion of the sleep cycle spent in REM sleep increases with each subsequent cycle over the course of a night. (Note that stages 3 and 4 are now considered to be one stage, stage 3.)
A sleeper first enters REM sleep after about an hour and a half of sleep, and then the phase will last only briefly. However, for each successive sleep cycle, the proportion of the cycle spent in REM sleep increases, up to an hour long in later cycles. This is why it is important for humans to sleep in long stretches (e.g., once a day for 8 hours) rather than in short bursts (e.g., 4 times a day for 2 hours each time); we spend much less total time in REM sleep than we do when we get one long period of sleep, and we can’t get through the stages we need to heal and stay healthy.
Sleep Deprivation
Sleep deprivation tends to cause slower brain waves in the frontal cortex, shortened attention span, higher anxiety, impaired memory, and an unhappy mood. Conversely, a well-rested organism tends to have improved memory and mood. Research has demonstrated that some sleep stages are more important than others in achieving restfulness. For example, REM deprivation causes a significant increase in the number of attempts to go into the REM stage while asleep. On recovery nights, an individual will most likely move to stage 3 and REM sleep more quickly and experience "REM rebound," which refers to a large increase in the time spent in the REM stage. These findings are consistent with the idea that REM sleep is biologically necessary. REM sleep is thought to be largely responsible for the time that our brains take to consolidate learning and memory. Similarly, sleeping in short bursts is not sufficient for restfulness because it does not allow our bodies enough time for a complete cycle of sleep.