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No. 2037:

Today, why do we sleep? The University of Houston's College of Engineering presents this series about the machines that make our civilization run, and the people whose ingenuity created them.

In Handel's opera Semele, Cupid calls the Zephyrs to dance for Jupiter's love, the sleeping Semele. He tells them to fill her dreams with pleasure. And so, when she finally wakes she sings,

O sleep, why dost thou leave me,
Why thy visionary joys remove?
O sleep, again deceive me,
To my arms restore my wand'ring love!

That's one of our many ideas as to how sleep serves us. But sleep seldom provides such visionary joy. Good sleep is something we all want and must sometimes struggle to get. Coleridge was closer to the nub of it when his Ancient Mariner cried,

Oh sleep! it is a gentle thing, 
Beloved from pole to pole! 
To Mary Queen the praise be given!
She sent the gentle sleep from Heaven,
That slid into my soul. 

The Ancient Mariner, it turns out, was praising Slow Wave sleep, while Semele had awakened from REM, or Rapid-Eye-Movement sleep.

During Slow-Wave sleep our brains shut down, in four stages, to almost no activity. It's marked by very slow electrical brain waves. We dream during the shorter periods of REM sleep. We usually cycle into REM a few times during the night.

Writer Carl Zimmer tells about recent sleep research. After studying all kinds of creatures, scientists find a puzzling variability. Chimpanzees and fruit flies both sleep around ten hours a day. But Giraffes sleep less than two hours, while possums and brown bats sleep almost twenty. Where's the pattern?

Protection from predators is an important part of the story since a sleeping animal is vulnerable. Indeed, many bird species, and a few aquatic mammals, sleep on alternate sides of their brains, with a waking eye open. But for most, it's less dangerous to spend a shorter time in full sleep.

At this point, animal behaviorists suspect that deep sleep is a time for our brains and nervous system to relax -- a time when our synaptic connections loosen. REM is more puzzling. It's also highly variable. For you and me it's about a quarter of our sleep. For zebra finches, fruit flies, and burrowing owls, REM is almost nonexistent, while dogs and Asian elephants spend almost a third of their sleeping in the REM state. 

The purpose of REM is what's really puzzling. Some humans need no REM at all. Some are on antidepressants; some have suffered brain injuries. But others just appear to be wired that way; and none's the worse for being without it. One researcher speculates that REM is no more than a preparation for awakening.

Science is exciting only when it's about things we don't yet understand. And sleep has been one of the great puzzles throughout the ages. Dreams have been a mystery with a hundred explanations. Now, as we learn a little more, the mystery itself expands -- and the question becomes all the more tempting.

I'm John Lienhard, at the University of Houston, where we're interested in the way inventive minds work.

(Theme music)

C. Zimmer, Down for the Count. The New York Times, Science Times,Tuesday, Nov. 8, 2005, pp. D1 and D3.

The fragment of "O Sleep" in the audio track is sung by soprano Kathleen Battle in the recording of Semele with the English Chamber Orchestra and Ambrosian Chorus, Deutsche Grammophon 435-782-2.

Sleeping Cats
Cats sleep over 13 hours per day with just over one quarter of that time spent in REM sleep. Cats spend more time dreaming than we do.  (Photo by John Lienhard)