Awake 4
REM +i
Sleepstage>©(^) + 4 4 4 + 4 + 4
Hours of sleep
Figure 20.1. Progression during a night’s sleep through the various stages of
NREM and REM.
The amount of time spent in REM-stage sleep per night varies
across the life span. Newborn infants sleep perhaps sixteen hours a
day, with half of that sleep time spent in the REM stage. By the age of
one year, a child may be sleeping twelve hours a day, with four hours
of REM. The amount of time spent in REM decreases throughout child-
hood, stabilizes by puberty at around 1.5 to 2 hours per night, and re-
mains that way throughout adulthood.
Mammals have REM sleep. Birds have REM sleep. However, REM-stage
sleep has not been found in reptiles, amphibians, or fish. This suggests
that REM is a more recent evolutionary development, perhaps related
in some way to the complexity of the brain’s cerebrum. During REM
sleep clusters of cells in the pons and midbrain become active and
spread neural excitation into the cerebral cortex. The neurotrans-
mitter acetylcholine is central to this excitation, and many of these
excitatory REM-active neurons innervating the cortex are cholinergic.
The result is widespread activation of the cerebral cortex, producing