The NY Times has a nice article about how scientists are studying sleep in animals in order to better understand the mysteries of human sleep:
It has been almost 600 million years since human ancestors diverged from those of flies. As those ancestors evolved, their sleep evolved as well. Human sleep, for example, features not only slow-wave sleep, but bouts of sleep when the eyes make rapid movements and when we dream. Rapid eye movement, or REM sleep, as it is known, generally comes later in the night, after periods of intense slow-wave sleep.
Other mammals also experience a mix of REM and non-REM sleep, as do birds. Sleep researchers would like to know whether this pattern existed in the common ancestors of birds and mammals, reptilian animals that lived 310 million years ago. It is also possible that birds and mammals independently evolved this sleep pattern, just as birds and bats independently evolved wings.
Answering that question may help scientists understand why REM sleep exists. Scientists have long debated its function, suggesting that it may play important roles in memory or learning. In the Oct. 27 issue of Nature, Jerome Siegel, a sleep expert at the University of California, Los Angeles, argues that REM does not play a vital physiological role like slow-wave sleep. He points out that brain injuries and even medications like antidepressants can drastically reduce REM without any apparent ill effect.
"People who don't have REM sleep are remarkably normal," Dr. Siegel said. "There's no evidence for any intellectual or emotional problems."
So why do mammals and birds have REM sleep at all? "The best answer I can come up with is that it's there to prepare you for waking," Dr. Siegel said. "When the important work of sleep is done, REM sleep just makes you as alert as you can be while you're asleep."
Circadiana blogged about this topic on Nov 1st.