318 Insomnia
Figure 14.1 A sleep histogram illustrating the sequence of sleep stages for
a good nights• sleep in a young adult.Source:C. Morin (1993). Copyright
1993 by Guilford Press. Reprinted by permission of the publisher and author.
AWAKE
REM
1
2
3
SLEEP STAGES^4
1234
HOURS5678•delta,Ž or •slow-wave sleepŽ because of the presence of
slow EEG waves of high amplitude called delta waves. After
reaching stage 4, the EEG pattern reverses through stage 3,
stage 2, and “nally gives place to the “rst REM sleep
episode.
In REM sleep, the EEG pattern is very similar to that ob-
served in stage 1. Brain waves of low-amplitude and high fre-
quency are, however, accompanied by rapid movements of
the eyes under the lids. The REM stage is often referred to as
•paradoxical sleepŽ because it is characterized by a loss of
core muscle tone while the activity in the brain and in the
autonomic system are at a level similar to that seen in wake-
fulness. Apart from occasional muscle twitches, the body is
essentially paralyzed during this stage. The most vivid
dreams occur during REM sleep, even though dreamlike ac-
tivity may also be recalled when subjects are woken from the
NREM stages.
In healthy adults with a regular sleep schedule, the pro-
portion of time spent in REM sleep is about 25% and in
NREM sleep 75%. NREM Stage 1 represents about 5%,
Stage 2 another 50%, and Stages 3 to 4 about 20%. The dis-
tribution of these stages follows a very organized sequence
(see Figure 14.1), with slow-wave sleep occurring mainly in
the “rst third of the night and REM sleep becoming more
prominent and more intense in the latter part of the night or
early morning hours.
Biopsychosocial Determinants of Sleep
Circadian and Homeostatic Factors
The propensity to sleep and the type of sleep experienced are
very dependent on circadian factors. Sleep is just one of
many biological (e.g., body temperature, growth hormone
secretion) and behavioral functions (e.g., meal schedules, so-
cial interactions) that are regulated by circadian rhythms.
Internal brain-based mechanisms (located in the hypothala-
mus), or biological clocks, regulate this alternation between
different states while interacting closely with time cues pro-
vided by the environment. The light-dark cycle is the most
important of these cues (Parkes, 1985). Social interactions,
work schedules, and meal times are other extrinsic time cues
that also contribute to regulating our sleep-wake cycles.
Homeostatic factors can also impact signi“cantly on sleep.
For instance, the time to fall asleep is inversely related to the
duration of the previous period of wakefulness. With pro-
longed sleep deprivation, there is an increasing drive to sleep.
Upon recovery, there is a rebound effect producing a shorter
sleep latency, increased total sleep time, and a larger propor-
tion of deep sleep (Webb & Agnew, 1974).
Daily variations in core body temperature, which are also
controlled by circadian factors, are closely tied to sleep-wake
patterns. At its lowest point in the early hours of the day (e.g.,
3:00 to 5:00A.M.), body temperature starts to increase near the
time of awakening and peaks in the evening. Alertness is at its
maximum during the ascending slope of the body temperature
curve. In contrast, sleepiness and sleep itself occur as temper-
ature decreases. In the absence of time cues or any constraint,
individuals tend to choose a bedtime that is closely linked to a
decrease in body temperature, while awakening occurs
shortly after it begins to rise again (Monk & Moline, 1989).
These basic facts about homeostatic and circadian princi-
ples have important implications for understanding problems
sleeping as well as problems staying awake. For night-shift
workers, even those who sleep well during the day, it is often
very dif“cult to stay alert around 3:00 or 4:00 A.M. because of
decreased body temperature at that time. For the same reason,
truck accidents on the road are proportionally more frequent
during early morning hours, despite less dense traf“c during
these hours. Conversely, for insomniacs, their body tempera-
ture tends to remain elevated throughout the night, explaining
partly why they have dif“culties sleeping.Age and MaturationImportant changes in the pattern of sleep accompany the nat-
ural maturation process that occurs throughout the life span.
Newborns sleep about 16 to 18 hours in short episodes dis-
tributed throughout the day and the night, with REM sleep
occupying more than 50% of total sleep time. From early
childhood to late adolescence, the sleep architecture becomes
progressively more organized into a single nocturnal phase.
Total sleep time decreases gradually to level off in early
adulthood at an average of 7 to 9 hours per night. There are
individual differences in sleep needs, probably determined
genetically, with the average being around 8 hours. Sleep
changes occur very gradually during adulthood, with a