Stages of Sleep

Learning Objectives

  • Differentiate between REM and non-REM sleep
  • Describe the stages of sleep

Sleep is not a uniform state of being. Instead, sleep is composed of several different stages that can be differentiated from one another by the patterns of brain wave activity that occur during each stage. These changes in brain wave activity can be visualized using EEG and are distinguished from one another by both the frequency and amplitude of brain waves. Sleep can be divided into two different general phases: REM sleep and non-REM (NREM) sleep. Rapid eye movement (REM) sleep is characterized by darting movements of the eyes under closed eyelids. Brain waves during REM sleep appear very similar to brain waves during wakefulness. In contrast, non-REM (NREM) sleep is subdivided into three stages distinguished from each other and from wakefulness by characteristic patterns of brain waves. The first three stages of sleep are NREM sleep, while the fourth and final stage of sleep is REM sleep. In this section, we will discuss each of these stages of sleep and their associated patterns of brain wave activity.

[Note that psychologists originally identified four stages of non-REM sleep, but these were revised in 2008, resulting in just three distinct phases of NREM sleep. You will see that stage 3 of NREM sleep is sometimes presented as both stage 3 and stage 4 in various texts.]

NREM Stages of Sleep

The first stage of NREM sleep is known as stage 1 sleep. Stage 1 sleep is a transitional phase that occurs between wakefulness and sleep, the period during which we drift off to sleep. During this time, there is a slowdown in both the rates of respiration and heartbeat. In addition, stage 1 sleep involves a marked decrease in both overall muscle tension and core body temperature.

In terms of brain wave activity, stage 1 sleep is associated with both alpha and theta waves. The early portion of stage 1 sleep produces alpha waves, which are relatively low frequency (8–13Hz), high amplitude patterns of electrical activity (waves) that become synchronized. This pattern of brain wave activity resembles that of someone who is very relaxed, yet awake. As an individual continues through stage 1 sleep, there is an increase in theta wave activity. Theta waves are even lower frequency (4–7 Hz), higher amplitude brain waves than alpha waves. It is relatively easy to wake someone from stage 1 sleep; in fact, people often report that they have not been asleep if they are awoken during stage 1 sleep.

EEG Recordings During Sleep showing shape of brain waves at four different stages of sleep and, for comparison, brain waves while awake. Each sleep stage has associated wavelengths of varying amplitude and frequency. Relative to the others, “awake” has a very close wavelength and a medium amplitude. Stage 1 (NREM Alpha) is characterized by a generally uniform wavelength and a relatively low amplitude which doubles and quickly reverts to normal every 2 seconds. Stage 2 (NREM Theta (sleep spindles; k-complexes)) is comprised of a similar wavelength as stage 1 but lower amplitude. It introduces the K-complex from seconds 10 through 12 which is a short burst of doubled or tripled amplitude and decreased wavelength. Stage 3 (NREM Delta) shows a more uniform wave with gradually increasing amplitude. Finally, REM sleep looks much like stage 2 without the K-complex.

Figure 1. Brainwave activity changes dramatically across the different stages of sleep.

As we move into stage 2 sleep, the body goes into a state of deep relaxation. Theta waves still dominate the activity of the brain, but they are interrupted by brief bursts of activity known as sleep spindles (Figure 3). A sleep spindle is a rapid burst of higher frequency brain waves that may be important for learning and memory (Fogel & Smith, 2011; Poe, Walsh, & Bjorness, 2010). In addition, the appearance of K-complexes is often associated with stage 2 sleep. A K-complex is a very high amplitude pattern of brain activity that may in some cases occur in response to environmental stimuli. Thus, K-complexes might serve as a bridge to higher levels of arousal in response to what is going on in our environments (Halász, 1993; Steriade & Amzica, 1998).

A graph has an x-axis labeled “time” and a y-axis labeled “voltage. A line illustrates brainwaves, with two areas labeled “sleep spindle” and “k-complex”. The area labeled “sleep spindle” has decreased wavelength and moderately increased amplitude, while the area labeled “k-complex” has significantly high amplitude and longer wavelength.

Figure 3. Stage 2 sleep is characterized by the appearance of both sleep spindles and K-complexes.

Stage 3 of sleep is often referred to as deep sleep or slow-wave sleep because these stages are characterized by low frequency (up to 4 Hz), high amplitude delta waves (Figure 4). During this time, an individual’s heart rate and respiration slow dramatically. It is much more difficult to awaken someone from sleep during stage 3 than during earlier stages. Interestingly, individuals who have increased levels of alpha brain wave activity (more often associated with wakefulness and transition into stage 1 sleep) during stage 3 often report that they do not feel refreshed upon waking, regardless of how long they slept (Stone, Taylor, McCrae, Kalsekar, & Lichstein, 2008).

Polysonograph a shows the pattern of delta waves, which are low frequency and high amplitude. Delta waves are found mostly in stage 4 of sleep.

Figure 4. Delta waves, which are low frequency and high amplitude, characterize slow-wave stage 3 sleep.

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REM Sleep

As mentioned earlier, REM sleep is marked by rapid movements of the eyes. The brain waves associated with this stage of sleep are very similar to those observed when a person is awake, as shown in Figure 5, and this is the period of sleep in which dreaming occurs. It is also associated with paralysis of muscle systems in the body with the exception of those that make circulation and respiration possible. Therefore, no movement of voluntary muscles occurs during REM sleep in a normal individual; REM sleep is often referred to as paradoxical sleep because of this combination of high brain activity and lack of muscle tone. Like NREM sleep, REM has been implicated in various aspects of learning and memory (Wagner, Gais, & Born, 2001), although there is disagreement within the scientific community about how important both NREM and REM sleep are for normal learning and memory (Siegel, 2001).

Polysonograph with the period of rapid eye movement (REM) highlighted.

Figure 5. A period of rapid eye movement is marked by the short red line segment. The brain waves associated with REM sleep, outlined in the red box, look very similar to those seen during wakefulness.

If people are deprived of REM sleep and then allowed to sleep without disturbance, they will spend more time in REM sleep in what would appear to be an effort to recoup the lost time in REM. This is known as the REM rebound, and it suggests that REM sleep is also homeostatically regulated. Aside from the role that REM sleep may play in processes related to learning and memory, REM sleep may also be involved in emotional processing and regulation. In such instances, REM rebound may actually represent an adaptive response to stress in nondepressed individuals by suppressing the emotional salience of aversive events that occurred in wakefulness (Suchecki, Tiba, & Machado, 2012).

While sleep deprivation in general is associated with a number of negative consequences (Brown, 2012), the consequences of REM deprivation appear to be less profound (as discussed in Siegel, 2001). In fact, some have suggested that REM deprivation can actually be beneficial in some circumstances. For instance, REM sleep deprivation has been demonstrated to improve symptoms of people suffering from major depression, and many effective antidepressant medications suppress REM sleep (Riemann, Berger, & Volderholzer, 2001; Vogel, 1975).

It should be pointed out that some reviews of the literature challenge this finding, suggesting that sleep deprivation that is not limited to REM sleep is just as effective or more effective at alleviating depressive symptoms among some patients suffering from depression. In either case, why sleep deprivation improves the mood of some patients is not entirely understood (Giedke & Schwärzler, 2002). Recently, however, some have suggested that sleep deprivation might change emotional processing so that various stimuli are more likely to be perceived as positive in nature (Gujar, Yoo, Hu, & Walker, 2011). The hypnogram below (Figure 6) shows a person’s passage through the stages of sleep.

This is a hypnogram showing the transitions of the sleep cycle during a typical seven hour period of sleep. During the first hour, the person goes through stages 1,2, and 3. In the second hour, sleep oscillates between Stages 2 and 3 before attaining a 30-minute period of REM sleep. The third hour follows the same pattern as the second, but ends with a brief awake period. The fourth hour follows a similar pattern as the third, with a slightly longer REM stage. In the fifth hour, stage 3 is no longer reached. The sleep stages are fluctuating from 2, to 1, to REM, to awake, and then they repeat with shortening intervals until the end of the seventh hour when the person awakens.

Figure 6. This hypnogram illustrates how an individual moves through the various stages of sleep. Deeper NREM sleep occurs early on in the night, while the duration of REM sleep increases as the night progresses.

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Think It Over

Researchers believe that one important function of sleep is to facilitate learning and memory. How does knowing this help you in your college studies? What changes could you make to your study and sleep habits to maximize your mastery of the material covered in class?

Glossary

alpha wave: type of relatively low frequency, relatively high amplitude brain wave that becomes synchronized; characteristic of the beginning of stage 1 sleep
delta wave: type of low frequency, high amplitude brain wave characteristic of stage 3 and stage 4 sleep
K-complex: very high amplitude pattern of brain activity associated with stage 2 sleep that may occur in response to environmental stimuli
non-REM (NREM): period of sleep outside periods of rapid eye movement (REM) sleep
rapid eye movement (REM) sleep: period of sleep characterized by brain waves very similar to those during wakefulness and by darting movements of the eyes under closed eyelids
sleep spindle: rapid burst of high frequency brain waves during stage 2 sleep that may be important for learning and memory
stage 1 sleep: first stage of sleep; transitional phase that occurs between wakefulness and sleep; the period during which a person drifts off to sleep
stage 2 sleep: second stage of sleep; the body goes into deep relaxation; characterized by the appearance of sleep spindles
stage 3 sleep: third stage of sleep; deep sleep characterized by low frequency, high amplitude delta waves
theta wave: type of low frequency, low amplitude brain wave characteristic of the end of stage 1 sleep

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