Although almost all animals and even plants have regular cycles of activity and inactivity, only mammals and birds seem to have the kind of sleep we experience, with its several stages, including Rapid Eye Movement (REM) sleep.
Based on studies of animals in captivity, the world's champion sleeper appears to be the two-toed sloth, logging a total of 20 hours every 24-hour day. That's truly slothful compared to the elephant, which may sleep as little as three hours a day. Why the difference? One reason may be that in its natural habitat the sloth slumbers in the treetops, relatively safe from danger, while the elephant must be always alert for big cats lurking in the grasslands. Matching the elephant in wakefulness is the giraffe, followed by such creatures as the horse, sheep, cow, and guinea pig.
Man and mole tie at eight hours of sleep. Opossums and armadillos fall just short of the sloth's 20 hours, while hamsters, rats, spiny anteaters, jaguars, and pigs snooze for about half of every 24-hour day.
In the comfort of our homes dogs and cats, as well as other kinds of pets and domesticated animals, sleep longer and sounder than they would if they had to exist in the wild. Their need for wakefulness - to hunt for food and to avoid being eaten - is reduced. A housebound cat or dog may sleep a total of 13 or 14 hours a day.