Why Do People Sleep

148
Chapter 4 Consciousness
Why Do People Sleep?
People need a certain amount of uninterrupted sleep to function normally. In fact, most
living creatures sleep. In trying to understand why people sleep, psychologists have
studied what sleep does for us and how the brain shapes the characteristics of sleep
(Siegel, 2005).
Humans and almost all animals display cycles of
behavior and physiology that repeat about every twenty-four hours. This pattern is
called a circadian rhythm. (Circadian is pronounced “sir-KAY-dee-en.” It comes from
the Latin circa dies, meaning “about a day.”) Longer and shorter rhythms also occur,
but they are less common.
Circadian rhythms are linked to signals such as the light of day and the dark of night
(Ohta, Yamazaki, & McMahon, 2005). However, most of these rhythms continue even
when no time signals are available. Volunteers living for months without external light
and dark cues maintain daily rhythms in sleeping and waking, eating, urination, hormone release, and other physiological functions. Under such conditions, these cycles
repeat about every twenty-four hours (Czeisler et al., 1999).
Disruption of the sleep-wake cycle can create problems. For example, air travel across
several time zones often causes jet lag, a pattern of fatigue, irritability, inattention, and
sleeping problems that can last for several days. The traveler’s body feels ready to sleep
at the wrong time for the new location. It tends to be easier to stay awake longer than
usual than to go to sleep earlier than usual. That is the reason that the symptoms of
jet lag are usually more intense after a long eastward trip (when time is lost) than after
a long westward journey (when time is gained; Lemmer et al., 2002). Symptoms similar to those of jet lag also appear in workers who repeatedly change between day and
night shifts and in people who try to go to sleep early on a Sunday night after a weekend of later-than-usual bedtimes (Czeisler et al., 2005; Di Milla, 2006). For these people,
Monday morning “blues” may actually be symptoms of a disrupted sleep-wake cycle
(Yang & Spielman, 2001).
The length of circadian sleep rhythms can vary from person to person such that
some have a natural tendency to stay up later at night (“owls”) or to wake up earlier
in the morning (“larks”). But because our sleep-wake rhythms stay about the same even
without external cues about light and dark, we must have an internal “biological clock”
that keeps track of time. This clock is in the suprachiasmatic nuclei (SCN) of the hypothalamus, as shown in Figure 4.5. The SCN receives light information from a special
set of photoreceptors in the eyes and then sends signals to hindbrain areas that initiate sleep or wakefulness (Albus et al., 2005; Lee et al., 2003; Saper, Scammell, & Lu,
2005). When animals with SCN damage receive transplanted SCN cells, their circadian
rhythms become like those of the donor animal (Menaker & Vogelbaum, 1993). SCN
neurons also regulate the release of the hormone melatonin. Melatonin, in turn,
appears to be important in maintaining circadian rhythms (Beaumont et al., 2004;
Cardinali et al., 2002). In fact, many of the symptoms associated with jet lag and other
disruptions in sleep-wake cycles can be prevented or treated by taking melatonin
(Revell & Eastman, 2005).
Sleep as a Circadian Rhythm
circadian rhythm A cycle, such as waking and sleeping, that repeats about
once a day.
jet lag Fatigue, irritability, inattention,
and sleeping problems caused by air
travel across several time zones.
The Functions of Sleep Examining the effects of sleep deprivation may help
explain why people sleep at all. People who go without sleep for as long as a week usually don’t suffer serious long-term effects. However, extended sleeplessness does lead to
fatigue, irritability, and inattention (Drummond et al., 2000; Smith & Maben, 1993).
Even short-term sleep deprivation—a common condition among busy adolescents and
adults—can also take its toll (Arnedt et al., 2005; Heuer et al., 2004; Stapleton, 2001).
For example, serious mistakes in patient care are more likely when medical interns work
sleep-disrupting extended hospital shifts than when they work more normal hours
(Landrigan et al., 2004). Most fatal car crashes in the United States occur during the
“fatigue hazard” hours of midnight to 6 A.M. (Coleman, 1992), and sleepiness resulting
149
Sleeping and Dreaming
FIGURE
4.5
Sleep, Dreaming, and the Brain
This diagram shows the location of some
of the brain structures thought to be involved in sleep and dreaming, as well as in
other altered states discussed later in the
chapter. For example, one area near the
suprachiasmatic nuclei acts as a “master
switch” to promote sleep (Saper, Chou, &
Scammell, 2001). If it is damaged, sleep
may be nearly impossible. Another nearby
area promotes wakefulness; individuals
with damage to this area sleep virtually all
the time (Salin-Pascual et al., 2001).
Pineal gland
Suprachiasmatic
nuclei
Locus
coeruleus
Cerebellum
Hindbrain
from long work shifts or other causes is a major factor in up to 25 percent of all
auto accidents (Barger et al., 2005; Garbarino et al., 2001; Philip et al., 2001). The fact
that “sleepy driving” can be as dangerous as drunk driving has led at least one U.S.
state (New Jersey) to expand the definition of reckless driving to include “driving while
fatigued” (i.e., having had no sleep in the previous 24 hours). Fatigue also plays a role
in many injuries suffered by sleepy young children at play or in day care (Valent,
Brusaferro, & Barbone, 2001). Learning, too, is more difficult after sleep deprivation;
but certain parts of the cerebral cortex actually increase their activity when a sleepdeprived person faces a learning task, so we are able to compensate for a while
(Drummond et al., 2000).
Scientists are looking for drugs that can combat the effects of sleep deprivation
(Porrino et al., 2005), but there appears to be no substitute for sleep itself. Some
researchers suggest that sleep helps restore the body and the brain for future activity and
helps to consolidate memories of newly learned facts (Gais & Born, 2004; Stickgold,
2005; Orban et al., 2006; Wagner et al., 2004). This restorative function is especially
associated with non-REM sleep, which would help explain why most people get their
non-REM sleep in the first part of the night (see Figure 4.4).
There is also an apparent need for REM sleep. For example, after total sleep deprivation, people don’t need to make up every hour of lost sleep. Instead, they sleep about 50
percent more than usual, then wake up feeling rested. But their “recovery” night includes
an unusually high percentage of REM sleep (Feinberg & Campbell, 1993). And if people
are deprived only of REM sleep, they compensate even more directly. In one study, participants were awakened whenever their EEGs showed REM. When allowed to sleep normally the next night, they “rebounded,” nearly doubling the percentage of time spent in
REM (Dement, 1960). This research suggests that REM has its own special functions. What
those functions might be is still unclear, but there are several interesting possibilities.
First, REM may improve the functioning of neurons that use norepinephrine (Siegel &
Rogawski, 1988). Norepinephrine is a neurotransmitter released by cells in the locus
coeruleus (pronounced “lo-kus seh-ROO-lee-us”; see Figure 4.5). During waking hours,
it affects alertness and mood. But the brain’s neurons lose their sensitivity to norepinephrine if it is released continuously for too long. Because the locus coeruleus is
almost completely inactive during REM sleep, researchers suggest that REM helps
restore sensitivity to norepinephrine and thus its ability to keep us alert (Steriade &
McCarley, 1990). Animals deprived of REM sleep show unusually high norepinephrine
levels and decreased daytime alertness (Brock et al., 1994).