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Sleep

Types of Sleep: Overview

Most of us have often ask “Why do we sleep?” Philosophers have pondered this question for centuries, yet despite decades of research scientists are still striving for an answer. Our understanding of sleep has certainly evolved since the great minds of Plato or Aristotle, who conceived of sleep as resulting from physiological processes such as the digestion of food (Barbera, 2008).

Sleep can be described as a reversible behavioral state of perceptual disengagement from and unresponsiveness to the environment.

Beginning in the 19th century, empirical studies of the time-course of memory inadvertently pointed toward a special role of sleep in reducing forgetting Opens in new window, but the implications of such findings were later seriously considered, sleep was proposed to play only a passive role in reducing memory interference from external, competing stimuli (Jenkins and Dallenbach, 1924; Van Ormer, 1933).

Fortunately, technological advances such as the electroencephalogram (EEG) led to the blossoming of more objective sleep research (Dement and Vaughan, 2000), allowing scientists to better understand this unique feature of life, including its active role in memory formation and the specific neural processes underlying this function (See How the sleeping brain impacts memory Opens in new window).

Stages of Sleep

The technological advances that led to the first all-night sleep recording in 1953 resulted in the classification of the various stages of sleep still recognized today.

Electrical signals from the brain (electroencephalogram; EEG), eyes (electrooculogram; EOG), and muscles (electromyogram; EMG), collectively making up the polysomnograph (PSG) recording, revealed highly similar patterns of electrical activity in the brain across individuals as they slept. It soon became clear that this pattern included several distinct sleep stages.

  1. NREM

The first type of sleep became known as NREM or non-rapid-eye-movement sleep and included four stages (1,2,3, and 4) demarking progressively deepening stages of sleep.

  1. REM

The second type of sleep, REM or rapid-eye-movement sleep, could also be broken down into two different types, known as tonic and phasic events. Tonic REM was characrerized by a lack of eye movements and reduced muscle tone, while phasic REM was characterized by rapid eye movements and minor muscle twitches (Carskadon and Dement, 1989).

Early observations of such recordings allowed researchers to map out the typical night of sleep, which is similar across individuals. At sleep onset, the brain first advances consecutively through stages 1-4.

Stages 3 and 4 together comprise slow-wave sleep (SWS), which is the deepest stage during the sleep cycle and is characterized by high-amplitude, low-frequency EEG patterns. The brain then shifts to REM sleep, which is characterized by low-amplitude, high-frequency EEG patterns, periodic rapid eye movements (REMs), and a drastic reduction in muscle tone.

Figure X Histogram depicting a typical night of sleep. Credit: ResearchGate Opens in new window
sleep histogram

Interestingly, the EEG patterns during REM sleep look very similar to patterns obtained during wakefulness, which is why the measurement of eye movements and muscle tone is required to confidently differentiate REM sleep from wakefulness; the EEG alone is often not sufficient.

Following REM sleep, the brain then cycles back through all the stages, repeating this cycle about every 90 minutes throughout the night. However, the distribution of these stages across a night sleep is not entirely even. The first half of the night contains a majority of the night’s SWS, whereas the amount of REM in the second half of the night is nearly doubled from the first (Figure X; Payne, 2011).

It should also be noted that, as of 2007, the American Academy of Sleep Medicine (AASM) has recommended an alteration to these customary sleep stages. According to this change, stage 1 and 2 remain the same, but are referred to as stages N1 and N2, respectively.

However, stages 3 and 4, which were once distinct, are combined into a single stage, noted as N3. Finally, REM sleep is simply referred to as stage R (Iber et al., 2007). Although these changes are recommended and are taught to those currently learning sleep scoring, some researchers still prefer to use the previous method of sleep scoring.

See also:
Adapted from: The Wiley Handbook on the Cognitive Neuroscience of Memory, First Edition. Edited by Donna Rose Addis, Morgan Barense, and Audrey Duarte. © 2015 John Wiley & Sons, Ltd. Published 2015 by John Wiley & Sons, Ltd.

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