146 CHAPTER 4
N1: LIGHT SLEEP As theta wave activity increases and alpha wave activity fades
away, people are said to be entering stage N1 sleep, or light sleep. Several rather inter-
esting things can happen in this stage of sleep. If people are awakened at this point,
they will probably not believe that they were actually asleep. They may also experi-
ence vivid visual events called hypnogogic images or hallucinations (Kompanje, 2008;
Mavromatis, 1987; Mavromatis & Richardson, 1984; Vitorovic & Biller, 2013). (The
Greek word hypnos means “sleep.”) Many researchers now believe that people’s expe-
riences of ghostly visits, alien abductions, and near-death experiences may be most
easily explained by these hallucinations (Kompanje, 2008; Moody & Perry, 1993). For
more about hypnogogic experiences and the role they may play in “hauntings,” see the
Applying Psychology section at the end of this chapter.
A much more common occurrence is called the hypnic jerk (Cuellar et al., 2015;
Mahowald & Schenck, 1996; Oswald, 1959). Have you ever been drifting off to sleep
when your knees, legs, or sometimes your whole body gives a big “jerk”? Although
experts have no solid proof of why this occurs, many believe that it has something to do
with the possibility that our ancestors slept in trees: The relaxation of the muscles as one
drifts into sleep causes a “falling” sensation, at which point the body jerks awake to pre-
vent the “fall” from the hypothetical tree (Coolidge, 2006; Sagan, 1977).
N2: SLEEP SPINDLES As people drift further into sleep, the body temperature con-
tinues to drop. Heart rate slows, breathing becomes more shallow and irregular, and
the EEG will show the first signs of sleep spindles, brief bursts of activity lasting only a
second or two. Theta waves still predominate in this stage, but if people are awakened
during this stage, they will be aware of having been asleep.
N3: DELTA WAVES ROLL IN In the third stage of sleep, the slowest and largest waves
make their appearance. These waves are called delta waves. These waves increase
during this stage from about 20 percent to more than 50 percent of total brain activity.
Now the person is in the deepest stage of sleep, often referred to as slow-wave sleep
(SWS) or simply deep sleep (Carskadon & Dement, 2011).
It is during this stage that growth hormones (often abbreviated as GH) are released
from the pituitary gland and reach their peak. The body is at its lowest level of func-
tioning. Eventually, the delta waves become the dominant brain activity for this stage
of sleep. See Figure 4. 3 , which shows progression, including brain activity, through the
sleep stages throughout one night.
People in deep sleep are very hard to awaken. If something does wake them, they
may be very confused and disoriented at first. It is not unusual for people to wake up in
this kind of disoriented state only to hear the crack of thunder and realize that a storm
has come up. Children are even harder to wake up when in this state than are adults.
Deep sleep is the time when body growth occurs. This may explain why children in
periods of rapid growth need to sleep more and also helps explain why children who
are experiencing disrupted sleep (as is the case in situations of domestic violence) suffer
delays in growth (Gilmour & Skuse, 1999; Saper et al., 2001; Swanson, 1994).
The fact that children do sleep so deeply may explain why certain sleep disorders
are more common in childhood. Indeed, many sleep disorders are more common in boys
than in girls because boys sleep more deeply than do girls due to high levels of the male
hormone testosterone (Miyatake et al., 1980; Thiedke, 2001).
R: RAPID EYE MOVEMENT After spending some time in N3, the sleeping person will
go back up through N2 and then into a stage in which body temperature increases
to near-waking levels, the eyes move rapidly under the eyelids, the heart beats much
faster, and brain waves resemble beta waves—the kind of brain activity that usually
signals wakefulness. The person is still asleep but in the stage known as rapid eye
movement sleep (R) and sometimes referred to as paradoxical sleep.
