Well, sleeping makes you feel considerably less sleepy. Aside from that, though, we're not really sure why we sleep. We (that is, the scientific community and I) think that sleep might have originated to protect the sleeper from predators by reducing their movements when they're vulnerable (nighttime, for most humans). Research has shown that REM and NREM sleep may serve specific biological functions. Sleep deprivation studies reveal that humans and other animals respond to sleep loss in the same way. When study subjects are deprived of REM sleep, they tend to spend longer periods in REM sleep during their next sleeping period to make up for the loss. REM sleep after deprivation is more intense, with more eye movements per minute than in normal REM sleep. Similarly, subjects deprived of NREM sleep usually spend more time in NREM sleep afterward. EEGs measuring brain activity show that this rebound NREM sleep also differs from normal NREM sleep. This research suggests that the body needs adequate levels of both REM and NREM sleep. Thus, NREM and REM sleep must serve different functions…or else someone's playing a really elaborate joke on the world's scientists.
Stages of Sleep
Functions of Sleep