called “sleep spindles,” which are waxing and waning brain waves. After 20–30
minutes, people sink into the third stage, delta sleep. Named after the regular, slow
brain waves that are characteristic of this stage of quiet sleep, delta sleep lasts about
30–40 minutes, during which the muscles are relaxed, although most people make
major postural adjustments every 5–20 minutes. Some dreaming occurs during delta
sleep, but it is poorly recalled, not vivid or emotional, and generally pleasant. The
sleep pattern then retraces the same stages in reverse order.
About 90 minutes after the onset of sleep, several abrupt physiological changes
occur as the sleeper enters REM sleep, or the active state, for the first time of the
night. It is the sleep phase of vivid, memorable dreaming. Brain waves become
desynchronized in a fast activity pattern that is similar, but not identical, to that of
the waking state. An accompanying profound loss of muscle tone throughout the
body causes a general paralysis except for the muscles of the eyes, middle ear ossi-
cles, and respiration. Sometimes people awaken partially from REM sleep before the
paralysis fades away, so that their body is still paralyzed, though they are otherwise
awake. Sleep paralysis, as this state is called, can occur as people are falling asleep
(rarely) or waking up (more frequently). Although the sensation may be terrifying,
especially at the first occurrence, sleep paralysis is harmless.
During REM sleep, breathing may accelerate to a panting pace, and the rhythm of
the heart may speed up or slow down. Typically, men have erections and women
experience vaginal lubrication during every REM cycle, regardless of dream content.
It is not unusual for men to ejaculate and women to experience orgasm during this
time. During REM sleep, the cells of our brains, which have fired steadily while we
were awake, fire in a wild and erratic pattern. Some neuroscientists believe dream-
ing is the brain’s attempt to impose meaning on these signals from random firings.
During a typical night’s sleep, the average adult alternates between periods of REM
sleep and quiet sleep at regular intervals four to six times each night. After the first
REM period, the intervals between REM periods decrease throughout the night, while
the length of each REM period increases. REM sleep is both the deepest and lightest
stage of sleep. It is the stage when people are least likely to be aroused by environ-
mental stimuli and it is also the stage when people are most likely to awaken spon-
taneously.
REM sleep is a primary means of brain development and maturation. Infants born
10 weeks prematurely spend 80 percent of the total sleep time in REM sleep and
those born two to four weeks prematurely spend 60–65% in REM sleep. Full-term
newborns spend about half of their sleeping time in REM sleep, which decreases to
30–35% by the age of two. REM sleep stabilizes at about 25% by 10 years of age and
shows little change until people reach their 70s or 80s when it decreases to about230 ABSOLUTE BEGINNER’S GUIDE TOALTERNATIVE MEDICINE