Bodily Defenses Image Map

What Does the Brain Do?

The Brain Puts Us To Sleep

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 to sleep.

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 experimentally.

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 wakefulness:

1. True sleep only occurs in higher animals that have a relatively large brain that performs more than primitive basic functions.

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 wakefulness.

Evidence against the "rest idea" is that:

1. Most neurons do not fire less during sleep; some even fire more vigorously.

2. The most obvious change in sleep is a shift toward more slow-frequency oscillation and synchrony among cortical areas.

3. The brain is "working" in all stages of sleep. It  consolidates memories of the day's events.

Why Do We Dream?

Nobody knows. 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 wakefulness.

2. Dreams occur in which events seem to be consciously perceived and in which the dreamer is an  agent in the dream.

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 dream.

5. REM most likely arises from some of the same ascending brainstem arousal influences that create and sustain wakefulness.

6. REM shares many of the properties of wakefulness and thus may be a transitional state between sleep and wakefulness.




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