I also my info from encyclopedias and the internet. The most helpful was the encyclopedia. Maybe you would like to hear some of the more interesting facts I learned - I'll list some. There is a sleep disorder called sleep apnea that pauses in breathing occur during sleep, these pauses are 30 - 60 seconds long and cause loud snoring. This disorder has been fatal.
Scientists use an electroencephalograph to measure brain waves during sleep.
Extra sleep is a not a requirement. REM sleep is when dreaming occurs. During this stage the eye moves back and forth rapidly. Sleep researchers discovered this by waking people during REM sleep. An important issue about my topic is Sleep as an adaptive process.