Most animals follow a daily rhythm of rest and activity like us, but while all mammals and birds have REM and NREM sleep cycles, we don’t all do it the same way. For example, dolphins and whales can never ‘zonk out’ completely as they have to go to the surface regularly to breathe. To keep from drowning, only one half of their brain sleeps at a time.
Giraffes are so vulnerable while slumbering that they sleep only five minutes at a time, adding up to less than five hours a day.
At the other extreme, koalas spend five hours a day eating low- energy eucalyptus leaves, resulting in a slow metabolism that forces them to sleep for the other 19 hours!