Sleep is universal across animal species and emerged with the earliest forms of life on Earth. There are four differences in sleep found within and between species. The first is diversity in amount of sleep time. For example, animal species within the same family group might have remarkably different sleep needs, yet there are also species from different family groups that have similar needs. Evolution shaped these needs as part of a balancing act between waking survival demands, restoring physiological needs, and meeting community needs.  

The second difference is composition of sleep across species, meaning “not all species experience all stages of sleep” (60). NREM sleep appears universal across species, whereas mammals, except possibly for aquatic mammals, and birds are the only species in the animal kingdom that experience REM sleep. Given that mammals and birds evolved separately, the fact that REM sleep evolved in both lineages indicates its fundamental need. While NREM sleep is common across all species, this does not mean this sleep stage is more important than REM sleep. Sleep studies illustrate that after sleep deprivation, the brain tries to salvage both over several recovery nights. Unfortunately, no species can gain back all the sleep lost.