The Brain Puts Us
We say we "fall" asleep. But actually the brain puts us to sleep if
we are tired, bored, and unstimulated. The brain actually has circuits
("sleep centers") that, when stimulated under relaxed conditions, put us
||Areas of the brain (in red). when active in an
otherwise relaxed brain state, put us to sleep.
These areas may
coordinate as a "sleep system," but that has not been confirmed
Sleep as a Mixture of States
Early night's sleep is a deep sleep, where you "fall
into a deep pit." Off and on during the night, you have dreams (most of
which you do not remember). Toward early morning, dreaming increases,
and you typically wake up in a dream.
Left: Change in sleep states during a typical night's sleep. Yellow:
wakefulness; Green: dream sleep (called REM because rapid eye movements
occur). Other shades: different stages of regular sleep, as indicated by
associated brain-wave (EEG) changes on the right.
Right: Brain-wave changes (EEG) during different stages of sleep.
Note that pattern during REM is similar to that during wakefulness, even
though REM occurs when one is asleep.
Key Features of a Night's Sleep
- Deep sleep (stage IV) occurs soon after going to sleep and does
not recur later in the night.
- REM is delayed, occurs on and off, and increases in duration as
the night progresses.
- REM terminates in early morning just before waking.
|Why Do We Sleep?
Nobody knows. Evidence
that it is needed to help brain recuperate from long periods of
1. True sleep only occurs in higher animals that
have a relatively large brain that performs more than primitive
2. Such a brain has a high metabolic rate,
suggesting that periods of rest and recovery might be needed.
Glucose consumption does decrease in brain during regular sleep
(but not REM).
3. Ion distributions and neurotransmitter
systems may have to be regenerated after a period of
Evidence against the "rest idea" is that:
Most neurons do not fire less during sleep; some even fire more
2. The most obvious change in sleep is a shift
toward more slow-frequency oscillation and synchrony among
3. The brain is "working" in all stages of sleep. It
consolidates memories of the day's events.
|Why Do We Dream?
Evidence that it is needed to help wake us up:
1. A normal
human nightly sleep is interrupted by episodes of brain
activation (REM) in which brain activity resembles that seen in
2. Dreams occur in which events seem to be consciously
perceived and in which the dreamer is an agent in the
3. SWS and REM have apparently co-evolved, being most
conspicuous in mammals.
4. Most people awaken after a normal night’s sleep in the
morning at some point in a REM episode, often in the midst of a
5. REM most likely arises from some of the same ascending
brainstem arousal influences that create and sustain
6. REM shares many of the properties of wakefulness and thus may
be a transitional state between sleep and wakefulness.