This is the 2007 version. Click here for the 2017 chapter 03 table of contents.

Attempts to Explain Sleep

Scientists are unsure why we sleep. Some point to the obvious advantage of staying hidden during vulnerable times. For primitive humans, the nighttime was more dangerous than the daytime. For nocturnal animals, the day is the time to hide. Perhaps the underlying reason for sleep is the simple fact that our planet goes through a radical change between day and night, and most animals are specialized for darkness or light but not both.

What are some theories about why animals sleep?

Sleep may help the body recover from extremes of activity. Marathon runners double their slow-wave (non-REM) sleep after running a marathon. However, sleep is not a rest for the brain as a whole. The brain consumes as much glucose and oxygen during sleep as it does when we are awake. It consumes slightly more than usual during REM sleep.

What is a hypnotoxin?

Is there a chemical that accumulates in the bloodstream while we are awake, causing us to become sleepy? Researchers have suspected that such a substance might exist since a researcher named Pieron showed in 1913 that blood from a sleeping animal would make another sleep as well. Such a substance is sometimes called a hypnotoxin. Perhaps such a substance is only cleared out of our systems as we rest. That would explain why we feel a need for sleep, and why we feel better afterward.

A chemical that seems to act like Pieron's hypnotoxin is the neurotransmitter adenosine. Adenosine accumulates when animals are awake. Adenosine-using neurons in the midbrain act to inhibit (shut down) neurons in the midbrain that arouse the cerebral cortex into activity. The more adenosine accumulates around these neurons, the sleepier an animal gets. On the other hand, when the animal goes to sleep, the adenosine is cleared away and the levels fall until the animal wakes up again (Rainnie, Grunze, McCarley, & Greene 1994). A role for adenosine in regulating alertness is plausible given what we know about caffeine and adenosine. Caffeine increases alertness, and caffeine blocks adenosine.

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