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Figure 11. Homeostatic regulation of sleep: the
pressure to sleep grows stronger across the day as
one stays awake and then dissipates when one
sleeps at night (shaded area). Sleep pressure
increases (dashed line) as one stays awake longer
into the normal sleeping hours.
reinforce the circadian cycle. We usually sleep
once daily because the homeostatic pressure to
sleep is hard to resist after about 16 hours, and
then while we sleep, our closed eyes block the
light signals to the biological clock. See Figure 11.
3.7. Dreams. An intriguing occurrence during
sleep is dreaming. Although reports of dreaming
are most frequent and vivid when an individual is
aroused from REM sleep, dreams do occur at sleep
onset and during NREM sleep as well. 23 During an
average night’s sleep, about two hours are spent
dreaming, mostly during REM sleep. Although
some dreams are memorable because of their
extraordinary or bizarre nature, other dreams
reflect realistic experiences. Despite this realism,
REM dreams are usually novel experiences, like a
work of fiction, instead of a replay of actual
events. Pre-sleep stimuli do not seem to affect
dream content. In fact, the source of the content
of any given dream is unknown. REM sleep and
dreams are associated with each other, but they
are not synonymous. While REM sleep is turned
on and off by the pons (see section 3.3 Sleep and
the brain, page 24), two areas in the cerebral hemispheres
(areas far from the pons that control
higher mental functions) regulate dreaming.
REM sleep and dreaming can be dissociated from
one another, as seen after the administration of
certain drugs or in cases of brain damage either to
the pons (loss of REM sleep but not of dreaming)
or to the frontal areas (no dreaming but REM
sleep cycle unaffected). Consequently, REM sleep
appears to be just one of the triggers for dreaming.
Using scanning techniques that assess brain activity,
scientists have determined which areas of the
4, 14
brain are active during REM sleep dreaming.
These areas are illustrated in Figure 12. Brain
regions that are inactive during dreaming include
those that regulate intelligence, conscious
thought, and higher-order reasoning. Higher-order
Figure 12. Areas of the
brain active during REM
sleep dreaming.
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Information about Sleep