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During this brief Stage 1 sleep, you may experience fantastic images resembling
hallucinations — sensory experiences that occur without a sensory stimulus. You may
have a sensation of falling (when your body may suddenly jerk) or of floating weightlessly.
These hypnagogic sensations (also called hypnic sensations) may later be incorporated
into your memories. People who claim aliens abducted them — often shortly after getting
into bed — commonly recall being floated off (or pinned down on) their beds (Clancy, 2005;
McNally, 2012). To catch your own hypnagogic experiences, you might use your alarm’s
snooze function.
You then relax more deeply and begin about 20 minutes of Stage 2 sleep, with its peri-
odic sleep spindles — bursts of rapid, rhythmic brain-wave activity that aid memory process-
ing (Studte et al., 2017). Although you could still be awakened without too much difficulty,
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you are now clearly asleep.
Then you transition to the deep sleep of Stage 3. During this slow-wave sleep, which
lasts for about 30 minutes, your brain emits large, slow delta waves and you are hard to
awaken. Have you ever said, “That thunder was so loud last night!” only to have a friend
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respond, “What thunder?” Those who missed the storm may have been in delta sleep. (It is
at the end of this stage that children may wet the bed.)
REM Sleep

About an hour after you first fall asleep, a strange thing happens. Rather than continu-
ing in deep slumber, you ascend from your initial sleep dive. Returning through Stage 2
(where you’ll ultimately spend about half your night), you enter the most intriguing sleep
phase — REM (R) sleep. For about 10 minutes, your brain waves become rapid and saw-
toothed, more like those of the nearly awake Stage 1 sleep. But unlike in Stage 1, during REM
sleep your heart rate rises, your breathing becomes rapid and irregular, and every half-minute
or so your eyes dart around in momentary bursts of activity behind closed lids. These eye
movements announce the beginning of a dream — often emotional, usually story-like, and
richly hallucinatory. Dreams aren’t real, but REM sleep tricks your brain into responding as if
they were (Andrillon et al., 2015). Because anyone watching a sleeper’s eyes can notice these
REM bursts, it is amazing that science was ignorant of REM sleep until 1952.
Except during very scary dreams, your genitals become aroused during REM sleep. You
may have an erection or increased vaginal lubrication, regardless of whether the dream’s
content is sexual (Karacan et al., 1966). Men’s common “morning erection” stems from the
night’s last REM period, often just before waking.
During REM sleep, your brain’s motor cortex is active, but your brainstem blocks its
messages. This leaves your muscles relaxed — so much so that except for an occasional
finger, toe, or facial twitch, you are essentially paralyzed. (This immobility may occa-
sionally linger as you awaken from REM sleep, producing the disturbing experience of
sleep paralysis [Santomauro & French, 2009].) Moreover, you cannot easily be awakened.
hallucinations false sensory
experiences, such as seeing REM sleep is thus sometimes called paradoxical sleep: The body is internally aroused, with
something in the absence of an waking-like brain activity, yet asleep and externally calm. We spend about 600 hours a
external visual stimulus. year experiencing some 1500 dreams, or more than 100,000 dreams over a typical life-
hypnagogic sensations time — dreams swallowed by the night but not acted out, thanks to REM’s protective
bizarre experiences, such as paralysis.
jerking or a feeling of falling The sleep cycle repeats itself about every 90 minutes for younger adults (with shorter,
or floating weightlessly, while more frequent cycles for older adults). As the night wears on, deep Stage 3 sleep grows
transitioning to sleep. (Also
called hypnic sensations.) shorter and disappears. The REM and Stage 2 sleep periods get longer (Figure 1.5-7).
By morning, we have spent 20 to 25 percent of an average night’s sleep — some
delta waves the large, slow
brain waves associated with deep 100 minutes — in REM sleep. In sleep lab studies, 37 percent of participants have reported
sleep. rarely or never having dreams that they “can remember the next morning” (Moore, 2004).
Yet even they, more than 80 percent of the time, could recall a dream after being awakened

96 Unit 1 Biological Bases of Behavior

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