What are sleep stages and why should you care?

Research Summary
Pascal Wallisch

We’ve all heard that it is important to get “deep REM sleep” for peak performance. This is nonsense. Deep sleep and REM sleep are two distinct and radically different sleep stages. But what are sleep stages to begin with and how are they defined?

For most of human history, sleep was seen as a progressive lack of motor activity – the less activity, the deeper the sleep was assumed to be, based on what was observable. Modern sleep apps have revived this notion, equating the lack of muscle activity with the deepness of sleep.

But this is a mistake. With the advent of EEG to measure electrical brain activity and the discovery of REM sleep, since the second half of the 20th century, sleep stages are primarily defined by their brain wave characteristics, although other factors, such as eye movements, muscle tone, breathing, and blood oxygenation also play a role. 

The way sleep scoring works is by recording these physiological signals (brain waves via EEG, muscle tone via EMG, eye movements via EOG,  breathing via respiratory belts, and blood oxygen saturation via oximetry) throughout the night, then categorizing successive 30 second snippets (an “epoch”) by a clinician, according to agreed upon guidelines. In terms of brain waves, these consist mostly of considerations about amplitude (height) and frequency (length) of these waves during the epoch. On this basis, we distinguish 6 stages: Wake and 5 sleep stages – 4 nonREM (NREM) and one REM stage. The 4 NREM stages further divide into light sleep (1-2) and deep sleep (3-4).

A typical night for a healthy individual quickly goes into deep sleep, followed by a period of brief REM sleep. This is followed by 4-5 of cycles of NREM/REM sleep, each lasting for about 90 minutes. Moreover, a normal night is dominated by deep sleep early on, but REM sleep becomes more common as the night goes on.

Sleep stages seem to have different functions. Deep sleep seems to be the most refreshing stage (and is so important that it happens early in the night) whereas REM sleep seems to be most closely associated with creativity. REM sleep is also when most of the dreaming happens.

Sleep stages allow clinicians to detect abnormal sleep by creating a “sleep profile” for an individual and comparing it to what one would expect from a healthy individual. For instance, the sleep of an insomniac might be characterized by frequent wakings, or taking a long time to actually fall asleep. The profile of someone suffering from apnea might be characterized by a lack of deep sleep in addition to the breathing problems. Someone with REM sleep at the beginning of the night (or “sleep onset REM”) might be suffering from narcolepsy, someone with excessive REM sleep might be suffering from depression. The density of both REM and deep sleep decreases with age.

Sleep deprivation changes the sleep profile. If one consistently doesn’t get enough sleep (which tends to cut off REM sleep, as it mostly happens later in the night), “REM sleep pressure” builds up and can be seen earlier and earlier in the night. This reinforces the notion that there is indeed a lot going on inside your head at night – whether you are aware of it or not – and it is worth taking care of.



Top panel: Sleep profile during a typical night as scored by Rechtschaffen & Kales criteria, y-axis: Sleep stages (W(ake),R(EM),1,2,3,4. x-axis: Sleep duration in minutes.

Bottom panel. What the brain waves look like during the corresponding point in time of the upper panel, as seen on an oscilloscope, y-axis:Voltage. x-axis: Time in seconds.