In humans beyond the age of about 6 months, there are five subdivisions, or stages, of sleep (Borbély, 1986; Dement, 1976; Lavie, 1996; Lee-Chiong, 2008; Webb, 1975). When we first fall to sleep, we are in Stage 1. From there, we progress through Stages 2, 3, and 4, and then, after about one hour, enter a stage known as rapid-eye-movement (REM) sleep. Each stage has two major characteristics:

(a) a particular pattern of electrical activity in the brain;
(b) a particular cluster of mental and behavioral events.

We experience each of these five stages several times during the night (generally 3-5 times per night for adults). Just before you fall to sleep, your mind is relaxed and your thoughts drift, but your muscles still show a great deal of tension. Your EEG would show predominantly alpha waves at this time (see Section 2-5 for an illustration of the several types of brain waves discussed in this section; also see Figure 1 below). This waking period just before sleep onset is referred to as Stage 0 (zero is used because this period of relaxed wakefulness is not one of the five stages of sleep). You will spend about ten minutes or so in this stage before drifting into sleep. The point at which you first fall to sleep can be measured by sounding a soft tone and having you press a button each time the tone occurs. People who show brain activity suggesting that they are sleeping usually will stop pressing the button. You may go back and forth between Stage 0 and this stage of light sleep a few times before you stay asleep for good.

Stage 1—Light Sleep

At the onset of sleep, people show brain activity that consists primarily of theta waves (see Figure 1). During this stage, people can be awakened easily by quietly saying their names. Interestingly, when awakened, they often do not know that they were just sleeping (they may claim that they were just about to “fall to sleep"). Such subjective experiences suggest that Stage-1 sleep is a transitional stage between a waking state and a sleeping state. In adults, an episode of Stage 1 sleep lasts about five minutes and makes up about 2-5% of sleep during the night.

Figure 1. Electrical activity within the major stages of sleep as
recorded by the EEG (from Groves & Rebec, 1992, p. 428)

As they enter Stage 1, people may have hypnagogic experiences, which are unusual perceptual experiences that occur when transitioning from a waking to a sleeping state. A common hypnagogic experience is a sinking feeling in the stomach (as if one were falling) coupled with jerking awake. Just before and during the muscle contractions, a person may experience a vivid visual image. Other hypnagogic experiences involve seeing, hearing, or feeling something that is not there but that seems very real — that is, a visual, auditory, or tactile (touch) hallucination. People are more likely to have hypnagogic experiences when they are under stress or when they have changed their sleep schedules, but they can happen to anyone when going to sleep, and sometimes can even be quite frightening.

Hypnopompic experiences are unusual perceptual experiences that occur when transitioning from a sleeping to a waking state. When people are awakening, they may feel paralyzed (see this web site on sleep paralysis), feel a pressure on their chests, see or hear things that are not really there, and feel as if they are floating. These experiences also can be very frightening. Hypnagogic and hypnopompic experiences may help to explain reports of ghosts and visitations of extraterrestrial beings. For example, Blackmore, 1998,Nickell, 1995, and Reisner (2001) described the case of "John," a 36-year-old man who experienced hypnopompic hallucinations that he misinterpreted as night-time visitations by extraterrestrials. His sleep-related hallucinations, however, had begun many years before during 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)

At that time, John experienced an episode of severe depression with psychosis — a break with reality in which a person may experience hallucinations and delusional thinking — that may have been caused by his minterpretations of hypnopompic experiences. After being treated with medications for his psychotic depression, John eventually realized that he had not really seen demonic creatures or extraterrestrial beings but had experienced sleep-related hallucinations.

It is not known why people have such experiences, but one possibility is that hey are transitioning into or out of a dream state — that is, 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 1). 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. Although it is somewhat more difficult to awaken people from Stage-2 sleep, about one-third of them still are not aware that they were just sleeping when awakened. Delta waves also first appear during the second half of Stage-2 sleep, which shows that the cerebral cortex is becoming less active as sleep continues. In adults, an episode of Stage 2 sleep lasts between 10 and 25 minutes and makes up about 45-55% of sleep during the night.

Stage 3—Transitional Sleep

Delta waves first appear during Stage 2 but occur less than 20% of the time. When delta waves make up at least 20% of EEG activity, people are said to be in Stage 3. As delta waves become more common (as the electrical activity in the cortex becomes reduced), people become much harder to awaken. The first episode of Stage 3 lasts about five minutes and, in adults, lasts about five minutes.

Stage 4—Deep Sleep

When delta waves make up at least 50% of EEG activity, people are 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). The first episode of Stage 4 sleep generally lasts about 20 to 40 minutes. Stages 3 and 4 combined make up anywhere from 5-20% of sleep during the night, with younger adults getting more SW sleep than older adults.

During SW sleep (especially during Stage 4), people sometimes experience “sleepwalking” and “sleep terrors.” Sleepwalking (also called somnambulism) is defined as the expression of complex movements during sleep — movements that indicate some sort of purposeful action, even if the purpose is not obvious to an observer. For example, when I was a teenager, a friend slept over at my house and, during the night, he sat up, looked at me, and began moving his hand in the way that Bela Lugosi sometimes did in old movies when he was trying to hypnotize someone. A student of mine once provided the example of her boyfriend, who sometimes got up during the night and made himself a breakfast of “pancakes,” which actually consisted of dinner plates, and “syrup,” which consisted of peanut butter that he spread over the plates. Sleepwalking is seen commonly in children (typically between the ages of about 4 and 10 years) but only rarely in adults.

The popular belief that it is dangerous to awaken a somnambulist (a “sleepwalker”) seems to be false. In fact, it is much more dangerous not to awaken people who are sleepwalking: they have little awareness of their physical surroundings and, hence, may hurt themselves. Furthermore, there have been rare occurrences in which a person commits violent acts (including killing others) while sleepwalking (for example, see Automatism: the Sleepwalker's Defense). In this case, it might be dangerous to others not to awaken sleepwalkers (preferably from a safe distance).

A sleep (or night) terror is an abrupt awakening from SW sleep in which people (almost always children between about 4 and 10 years of age) feel and look terrified, often seem confused, and are unable to give a good reason for their terror. They may quickly go back to sleep and may not remember the event the next morning. It seems that neither people who are sleepwalking nor people who are exhibiting sleep terrors typically are dreaming during the episode (see below), although they may report that they saw or heard something.

REM Sleep

At the end of Stage-4 sleep, there is a relatively rapid rise through Stages 3 and 2, which takes about 5-10 minutes, into a fifth stage of sleep called rapid-eye-movement (REM) sleep. You can observe this stage yourself by watching a sleeping person’s eye movements, usually about 60 minutes after they fall asleep. When you see the eyes fluttering around in short bursts, the person is in REM sleep. The first episode of REM sleep lasts about 1-5 minutes.

EEG activity observed during REM looks very similar to beta waves— waves seen when a person is awake and alert— which seems to suggest that the person is wide awake. Nevertheless, people are asleep during REM and, in fact, may be very difficult to awaken. As you will see later, there are subtle differences between the EEG activity of people who are wide awake versus those who are in REM sleep.

In addition to beta-like activity, there are other interesting characteristics of REM sleep:

(a) Physiological processes of the body (for example, blood pressure and heart rate) begin to look more like those of a person who is awake.
(b) Muscle tone decreases dramatically so that the person, in essence, becomes paralyzed.
(c) Genital arousal often occurs (that is, erections occur in males and labial swelling in females, along with other signs of sexual arousal).
(d) Dreaming occurs frequently.

With respect to dreaming, when sleepers are awakened during REM sleep, about 85% of them report that they were just dreaming. When they are awakened during Stages 1-4 (referred to collectively as non-REM or NREM), they report fewer episodes of dreaming:

• Stage 1: about 50% of people report that they were just dreaming when awakened.
• Stage 2: about 10% of people report that they were just dreaming when awakened.
• Stage 3: there is little or no dreaming reported during this stage.
• Stage 4: there is little or no dreaming reported during this stage.

Nightmares may look like sleep terrors to an outside observer, but they are very different phenomena. A nightmare is a frightening dream that awakens a person and, because they are dreams, are most likely to occur during REM or Stage 1 sleep. Sleep terrors, on the other hand, rarely or never are associated with dreaming because they occur during SW sleep (usually during Stage 4). In a later section, we will look more closely at dreaming.

Individual Differences
The timing of the five sleep stages as well as the characteristics of each stage differ among people, and even over different nights for the same person. Some examples of these variations follow:

(1) People who are severely depressed are likely to quickly enter REM after first falling to sleep. This is called sleep-onset REM, which is defined as entering REM after spending 20 minutes or less NREM sleep Some people enter REM as they fall asleep, thereby skipping Stages 1-4 altogether, especially if they have been deprived of sleep for some time.
(2) People who are taking sleeping pills or who have consumed too much alcohol show less REM than others. 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.
(3) Some people may show a lack of muscle paralysis during REM sleep probably because the area of the brain that paralyzes the muscles is not functioning normally. When observed during REM sleep, these people often get up and begin performing complex behaviors, which seem to result from the acting out of their dreams. These behaviors look very similar to sleepwalking during Stages 3 and 4 but, in the case of REM movements, they are linked to dreaming (as already stated, there is virtually no dreaming during SW sleep). Anch, Browman, Mitler, and Walsh (1988) described one such case:

A 54-year-old businessman and church leader has been complaining of mild dizziness and headaches for 7 days. At 3:30 a.m., his wife is awakened from a sound sleep by his mumbling long tirades of horrid profanities. The man then sits up in bed with his eyes open, but not seeming to be focused on anything in the room. He clumsily hits at his wife three times, striking her only once in the face. He then lies down and returns to peaceful sleep. Similar events occur again at 5:00 a.m. During the following day, he is surprised to hear of his nocturnal behavior and shows no impairment in his waking activity. However, he begins to exhibit profane vocalizations and violent actions 3-4 times each night. (p. 87)

Another man, who also would try to hit his wife while he was sleeping, was found to be dreaming about an argument with a coworker.
(4) During afternoon naps, there are alterations of the timing of the five stages. For example, when napping, people enter REM more quickly and spend more time in it than they do at nighttime.
(5) The duration and timing of the five stages often change as a person ages. In the next section, some of the common differences among age groups are described.

Study Questions for Section 2-8

1. How are hypnagogic and hypnopompic experiences similar to and different from each other?
2. 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
   (e) REM
3. In which NREM stage or stages are people most likely to dream?
4. Is a person dreaming during a sleep terror? Why or why not?
5. Is a person dreaming while sleepwalking? Why or why not?
6. Is a person dreaming during a nightmare? Why or why not?
7. What distinguishes REM sleep from NREM sleep?
8. Who is most likely to sleepwalk?
9. Who is most likely to experience a sleep terror?
10. During which stage or stages are people least likely to realize that they were just sleeping when awakened?
11. What is "sleep-onset REM" and when is it most likely to occur?
12. Some people consistently use alcohol or sleeping pills to help them fall to sleep. Other than the possibility of addiction, why else may this not be a good idea?
13. What often happens when the relative absence of muscle tone characteristic of REM sleep does not occur?
14. What is a difference between afternoon naps and night-time sleeping in terms of the lengths of sleep stages?

Go to Quiz 2-8 questions

Go to Readings Section 2-9