The paper by Aserinsky and Kleitman (1953) showed that there are two main types of sleep: rapid-eye-movement (REM) sleep and non-rapid-eye-movement (NREM) sleep (also see Borbély, 1986; Dement, 1976; Lavie, 1996; Lee-Chiong, 2008; Webb, 1975). Some researchers have argued that these two types of sleep are so different from one other, and from the waking state, that consciousness itself can be thought of as consisting of into three fundamental states: the waking state, the REM state, and the NREM state (e.g., Hobson, 2009).
REM and NREM sleep is divided into five stages of sleep (Dement & Kleitman, 1957b; Rechtschaffen & Kales, 1968). Stages 1 through 4 make up NREM sleep1, whereas REM sleep generally is considered to consist of one stage. Each sleep stage is defined in terms of two major characteristics:
- a particular pattern of electrical activity in the brain (as measured by the EEG);
- a particular cluster of mental and behavioral events.
The four types of brain waves we will use to describe the stages of sleep are shown in Figure 1.
As you learned in Section 2-6, beta waves are associated with being awake and alert; alpha waves are associated with being awake but not trying to attend to anything; theta waves are associated with light sleep; and delta waves are associated with deep sleep.
Figure 2 shows the EEG activity associated with each of the five stages of sleep you will learn about in the rest of this section. (Note to PSY 101 students: On tests, you won’t be asked to identify waves by what they look like, but you will need to know what state of consciousness each wave is associated with).
Characteristics of NREM Sleep
Stage 0—Relaxed Wakefulness. Although it is not actually a stage of NREM sleep, the period just before you fall to sleep is sometimes referred to as Stage 0. During Stage 0, your mind is relaxed, your thoughts are drifting, and your muscles are tense. Your EEG at this point shows mostly alpha waves (see Figure 1), which often indicates a state of relaxed wakefulness. The point at which people fall to sleep can be measured experimentally by having them press a button each time they hear a tone. They stop pressing the button when they have fallen to sleep because they no longer are aware of the tone (Bonnet & Webb, 1979).
Stage 1—Light Sleep. At the onset of sleep, people show EEG activity consisting primarily of theta waves (see Figure 1). During this stage, people can be awakened easily (e.g., by quietly saying their names). When awakened from Stage 1, about two-thirds of people don’t realize that they were just sleeping; and they may even say that they were just about to “fall to sleep.” Such subjective experiences suggest that Stage-1 sleep is a transitional stage between the states of waking and sleeping. Stage 1 lasts about five minutes in young adults.
The onset of Stage-1 sleep sometimes is called hypnagogia (hypnos is Greek for sleep). People occasionally have strange and sometimes frightening psychological experiences at this time. Hypnogogic experiences are unusual perceptual experiences that occur when transitioning from a waking to a sleeping state, or vice versa (Schacter, 1976)2. A common hypnagogic experience is the sinking feeling you get in your stomach as if you’re falling, coupled with jerking awake (a hypnic jerk). Just before and during the muscle contractions, a person may experience a vivid visual image. Hypnagogic experiences also include hallucinations — seeing, hearing, smelling, tasting, or feeling something that is not really there.
You can have hypnogogic experiences anytime you are drowsy; that is, not just when you’re in bed but also when you’re engaged in a monotonous and boring activity, such as driving on an open highway. Here’s a description of the hypnogogic hallucinations experienced by a sleepy man while driving down a freeway late at night:
Last night, while on the freeway I saw what appeared as an electrical post falling onto the freeway. As it was falling it spun then hit the ground. It was so realistic that I actually swerved off to the side…. After seeing the post I saw two shadowy figures run across the freeway and immediately knew that wasn’t real after seeing that the post wasn’t there. Then I saw a black/shadowy figure of a bird that came and disappeared within seconds. It was heading toward my window then vanished. Finally when exiting the freeway I noticed a big cardboard box in the distance, except this time it looked real not shadowy. So I started to slow down to see if it was real or fake. I didn’t know what to believe anymore. As I got closer the image/object disappeared.
As you can see, hypnogogic experiences can be very frightening because they occur when a person feels as if he or she is wide awake but is actually in an altered state between waking and sleeping. Because people don’t realize that they actually are in an altered state, hypnogogia provides a good explanation of frightening visits by ghosts or demons (Furuya, et al., 2009; Nickell, 1995).
A hypnagogic experience that can be extremely frightening is sleep paralysis — a condition characterized by an inability to move, either when falling asleep or waking up (Conesa Sevilla, 2004). During an episode of sleep paralysis, people often experience one or more of the following:
- vivid hallucinations (such as those in the passage quoted above)
- choking sensations
- the feeling that their bodies are floating in the air
- the feeling of a pressure on their chests, as if someone is sitting on it
- sensations of being outside of their bodies (perhaps even looking back at their bodies)
- fear and anxiety that may become severe (feelings of terror or dread)
These experiences sound very similar to those reported by people claiming to have seen extraterrestrial beings at nighttime while they’re in bed (Clancy, 2005). For example, a 36-year-old man experienced hypnogogic hallucinations that he misinterpreted as night-time visitations by extraterrestrials (Blackmore, 1998). The man had experienced sleep-related hallucinations since childhood:
When he was five or six years old, while trying to get to sleep, he saw a little man in his room who was about six inches tall. John saw the little man go in and out of a door in the room. Later, at age twenty-six, after reading a popular book on UFO abduction (Strieber 1987), John woke up, unable to move, and saw a four-foot-tall, gray visitor, resembling the prototypic alien described in Strieber’s book. This led to some speculation on John’s part that he may have been abducted by aliens, perhaps more than once…. Under stress a few years later, John had more experiences of awakening in a paralyzed state and seeing a taller, dark and menacing visitor — one so tall it reached the top of a doorway. The apparition, moreover, wore a wide-brimmed black hat and black cloak, resembling characters from movies he had recently seen. (Reisner, 2001, p. 48)
John eventually experienced an episode of severe depression with psychosis. A psychosis is a psychological condition in which a person cannot distinguish reality from his own imaginings because of (in part) hallucinations and delusional thinking. After John was treated with medications for his psychotic depression, he realized that the demonic creatures and extraterrestrial beings he had seen were actually sleep-related hallucinations.
It isn’t known why people have such experiences, but a likely possibility is that they are transitioning between a waking state and a dreaming state — that is, that they are dreaming while also partially awake (see below).
Stage 2—Moderately Light Sleep. This stage is characterized by the appearance of “sleep spindles” and “K-complexes” (see Figure 3). A sleep-spindle is a rapid but short burst of electrical activity; a K-complex is a large fluctuation in voltage, which appears as a high peak and a deep valley on an EEG tracing. It is is more difficult to awaken people in Stage 2 than in Stage 1; but about one-third of people awakened during Stage 2 still aren’t aware that they were just sleeping. Delta waves, which are associated with deep sleep (see below), begin to appear during the second half of Stage-2 sleep, although they occur infrequently. The occurrence of delta waves indicates that the cerebral cortex, the outer part of the brain measured by the EEG, is becoming less active. Stage 2 lasts between 10 and 25 minutes in young adults.
Stage 3—Transitional Sleep. The amount of delta-wave activity increases near the end of Stage 2. When delta waves make up 20% of all EEG activity, a sleeping person is said to be in Stage 3. Electrical activity in the cortex is much lower than when the person first entered Stage 1, which means that it now is difficult to awaken the person. Stage 3 lasts about five minutes in young adults.
Stage 4—Deep Sleep. The amount of delta-wave activity increases throughout Stage 3. When delta waves make up 50% of all EEG activity, a sleeping person is said to be in Stage 4. Because delta waves are referred to as “slow waves,” Stages 3 and 4 together are called slow-wave sleep (abbreviated as SW sleep). Stage 4 lasts about 30 to 40 minutes in young adults. During SW sleep, especially during Stage 4, people sometimes experience episodes of “sleepwalking” or “sleep terrors.”
Sleepwalking (also called somnambulism) is defined as complex, seemingly purposeful movements during sleep. In other words, sleepwalking consists of behavior that seems to have a goal, even if that goal isn’t apparent to an observer, and which may not even involve getting out of bed. For example, when I was a teenager, a friend stayed at my house overnight. At one point during the night, he sat up, looked at me, and began moving his hand in the way that movie vampires of the 1930s sometimes did when hypnotizing a victim. Another example comes from a student: her boyfriend sometimes got up during the night and made himself a breakfast of “pancakes,” which actually were dinner plates; and “syrup,” which consisted of peanut butter that he spread over the plates. Parents of young children often have many examples because sleepwalking is seen most commonly in children between the ages of about four and ten years. Sleepwalking occurs rarely in adults, although some adults sleepwalk frequently.
In addition to complex purposeful behaviors, sleepwalkers may show one or more of the following:
- a blank facial expression;
- eyes that are fully open;
- sleep talking (which may consist of mumbling or of statements that don’t make sense);
- little or no awareness of the surroundings (e.g., bumping into objects).
If a sleepwalker wakes up during the sleepwalking episode, he or she may
- be confused or disoriented;
- become physically aggressive, often as a result of the confused or disoriented state;
- not remember the episode.
It can be dangerous if a person tries to wake up a sleepwalker and the sleepwalker becomes physically aggressive. However, the popular belief that it is dangerous to the sleepwalker to be awakened seems to be false. In fact, it probably is much more dangerous not to awaken sleepwalkers: they have little awareness of their physical surroundings and, therefore, may be harmed if they trip over furniture, fall down stairs, etc. Furthermore, there have been rare occurrences of sleepwalkers committing violent acts while asleep, including killing others (Siclari, Khatami, Urbaniok, et al., 2010; also see Ramsland, n.d.).
The following video shows that it isn’t only humans who sleepwalk — other mammals also sometimes do.
A sleep terror (or night terror) is an abrupt awakening from SW sleep in which a person looks terrified, may yell or scream, and seems confused. The person, who often cannot give a good reason for his emotional state, may immediately go back to sleep and, if asked about it the next morning, may not remember the episode. As with sleepwalking, sleep terrors occur primarily in children between the ages of four and ten years.
Neither sleepwalkers nor people people experiencing a sleep terror seem to be dreaming during the episode (see below).
Characteristics of REM Sleep
At the end of Stage 4, a relatively rapid rise occurs through Stages 3 and 2 (taking about 5-10 minutes) into a fifth stage of sleep called rapid-eye-movement (REM) sleep. You can determine when others are in this stage by watching their eye movements about 60-75 minutes after they fall asleep. When you see their eyes fluttering around in short bursts, the person is in REM sleep. At that time of the night, REM sleep lasts about five minutes.
The EEG activity of a person during REM sleep looks very similar to beta waves— waves seen when a person is awake and paying attention to things. However, people definitely are asleep during REM and, in fact, may be very difficult to awaken.
In addition to beta-like activity, REM sleep has other interesting characteristics:
- Physiological processes of the body (for example, blood pressure and heart rate) begin to look more like those of a person who is awake.
- Muscle tone decreases dramatically, to the point that the person is essentially “paralyzed.”
- Genital arousal usually occurs (e.g., erections in males and labial swelling in females).
- Dreaming occurs frequently.
Research on dreaming took a giant step forward when the link between REM sleep and dreaming was established (Dement & Kleitman, 1957b). About 85% of people awakened during REM sleep report that they were just dreaming. When awakened during NREM sleep, on the other hand, dreams are reported much less frequently, especially during deep sleep:
- Stage 1 — about 50% of people report that they were just dreaming
- Stage 2 — about 10% of people report that they were just dreaming
- Stage 3 — virtually no dream reports
- Stage 4 — virtually no dream reports
A nightmare is a frightening dream that awakens a person. Although nightmares may look similar to sleep terrors, they are very different phenomena. Nightmares are dreams, so they are most likely to occur during REM or Stage 1 sleep. On the other hand, sleep terrors occur during deep sleep (Stages 3 and 4), so they rarely or never are associated with dreaming. In a later section, we’ll look more closely at dreaming.
Both the timing of the five sleep stages and the characteristics of each stage differ among people; and they also differ over several nights for the same person. Here are some examples:
- Emotional problems. People typically enter REM sleep about 60-75 minutes after first falling to sleep. In sleep-onset REM, however, people enter REM 20 minutes or less after falling asleep. Sleep-onset REM often occurs in severely depressed people. In fact, they sometimes skip NREM sleep altogether and enter REM just as they fall asleep. This is most likely to occur in depressed people who haven’t slept much because of the insomnia that often is a part of severe depression..
- Drugs and alcohol. People taking sleeping pills or who have consumed too much alcohol show decreased amounts of REM sleep. In fact, if alcohol is abused over a long period of time, there may be such a severe reduction in REM sleep that the person feels fatigued and irritable much of the time.
- REM Sleep Behavior Disorder. Some people, typically older men, show a lack of muscle paralysis during REM sleep, and are diagnosed as suffering from REM Sleep Behavior Disorder (Schenck, Bundlie, Ettinger, & Mahowald, 1986). People suffering from this disorder act out their dreams during REM sleep. You’ll learn more about REM Sleep Behavior Disorder in Section 2-12.
- Time of day. During afternoon naps, the timing of the five stages — that is, how long it takes for each stage to begin after sleep onset and how long each stage lasts (its duration) — tends to change relative to their timing at night. For example, people enter REM more quickly during afternoon naps and spend more time in that stage.
- Age. The timing of the five stages changes as from infancy to old age. In the next section, some of the common differences among age groups will be described.
Study Questions for Section 2-8
- What is hypnogogia?
- What are some examples from your own life of hypnogogic experiences?
- How do we know when a person is in each of the following stages of sleep?
(a) Stage 1
(b) Stage 2
(c) Stage 3
(d) Stage 4
- In which NREM stage or stages are people most likely to dream?
- Is a person dreaming during a sleep terror? Why or why not?
- Is a person dreaming while sleepwalking? Why or why not?
- Is a person dreaming during a nightmare? Why or why not?
- What distinguishes REM sleep from NREM sleep?
- Who is most likely to sleepwalk?
- Who is most likely to experience a sleep terror?
- During which stage or stages are people least likely to realize that they were just sleeping when awakened?
- What is “sleep-onset REM” and when is it most likely to occur?
- Some people consistently use alcohol or sleeping pills to help them fall to sleep. With respect to sleep, why might this not be a good idea?
- What happens when the muscle paralysis characteristic of REM sleep doesn’t occur?
- What happens to the timing of sleep stages during afternoon naps?
Practice Quiz for Section 2-8
1 In 2007, the American Academy of Sleep Medicine decreased the number of sleep stages to four (Iber, Ancoli-Israel, Chesson, & Quan, 2007). But because most sleep research since the 1950s has used the 5-stage system, that is the way sleep will be classified in this textbook.
2 The transition from sleeping to waking sometimes is referred to as hypnopompia. Many people, however, find this confusing; and because the experiences during both types of transition are very similar or identical, we will use the term hypnagogia to refer to any transition involving sleeping and waking.
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