1. Why do we sleep?
Repair Theory: sleep serves a restorative function, allowing organisms to recuperate from physical, emotional, and intellectual demands

2. Support for Repair Theory
Common sense – tired people go to sleep and feel better afterwards
Growth hormone is secreted during stage 4 sleep (helps control metabolism, physical growth, and brain development)
After physical exertion, people have an increased amount of stage 4 sleep
Increased immune cell production during sleep
Important brain proteins created during REM sleep

3. Contradictions to Repair Theory
People can go long periods without sleep (record of 264 hrs) and not show any long-term damage
Some bodily functions that are not restorative increase during some stages of sleep (blood flow in the cerebral arteries, heart and respiration become irregular, brain activity is elevated)

4. Why do we sleep? An alternate theory …
Adaptive Theory:
Sleep has evolved as a means of conserving energy (needed for our high metabolism as warm-blooded species) and protecting individuals from predators, and is maintained through circadian rhythms

6. Support for Adaptive Theory
Sleep time varies across species depending upon vulnerability to attack, and time needed for food gathering
Animals sleep at different times of the day depending on their environment
Human sleep time decreased during the 20th century as our environments have changed

7. Contradictions to Adaptive Theory
Sleep interferes with some survival-enhancing activities
Does not fully address the need for different stages of sleep
Overall …
A combination of theories is necessary to partially explain the need for sleep!