Sleep in the form of regularly occurring periods of quiescence and some amount of sleep rebound can be found in even the simplest of organisms from earthworms and fruit flies to nonhuman primates and human beings. We do not see evidence, however, of the emergence of distinct sleep states until we come to the reptiles. Birds and aquatic mammals also evidence distinct sleep states including the phenomenon of unihemispheric sleep, which allows these animals to sleep while flying or swimming. REM may only occur bihemispherically. The presence of high voltage slow waves as well as REM-like brain activation patterns in reptiles, birds, and mammals suggests that the biphasic, REM, and NREM sleep phases we find in humans is a very ancient adaptation indeed and that its benefits outweigh the risks associated with quiescence and reduced responsiveness to the environment.