On the Function and Evolution of Sleep

On the Function and Evolution of Sleep

If physiologists devoted the most research time to behaviors humans engaged in the most, we would probably have a full understanding of the biological purpose of sleep. After all, humans, with the exception of most college students, spend one third of their lives in a somnolent state. Despite its fundamental role in human and animal life, sleep is, even in an age when neuroscience has reduced many behaviors to neurological mechanisms, still quite mysterious. What processes are taking place during sleep that benefit the organism? Why spend so much time in an unresponsive and vulnerable state? That these questions haven't been definitively answered is really not a function of a lack of effort on the part of scientists, but rather of the difficulties inherent in studying sleep. It is easy to observe the harm that is done to a human or animal deprived of sleep. A rat prevented from sleeping will lose the ability to maintain body temperature and die in about three weeks, showing no evidence of physiological damage (1). In humans, sleep deprivation impairs thinking and suppresses the immune system. But why this deterioration take place is less clear, and the object of disagreeing theories. This essay will try to explain the function of sleep based on what is now known, and attempt to shed light on the reasons and mechanisms for its evolution.

An investigation of the control, and perhaps the origin, of sleep might best begin with the hypothalamus, a flat, horizontal structure in the brain that is known to play an important role. Damage to the back portion of the hypothalamus causes somnolence, indicating that when intact, it sustains alertness (1). In contrast, damage to the front part indu...

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...t suggests that some of the circuits employed in sensory reception and processing also function to establish and maintain memory, and that both functions cannot be achieved simultaneously. Sleep circumvents this limitation, and is thus a well adapted behavior.

WWW Sources

1)Scientific American "In Focus"

http://www.sciam.com/missing.cfm

2)Sleep and Memory: Evolutionary Perspectives

http://bisleep.medsch.ucla.edu/SRS/publications.htm

3)REM Sleep = Dreaming: Only a Dream

http://bisleep.medsch.ucla.edu/SRS/rosenlicht-feinberg.htm

4)Birds May Refine Their Songs While Sleeping" ,Science Magazine article

http://www.sciencemag.org/cgi/content/full/282/5397/2163

Additional Resources

Discussion on Sleep Evolution ,Scientific American article

http://www.sciam.com/missing.cfm

The Journal "Sleep"

http://www.stanford.edu/dept/sleep/journal/