Mammals sleep as little as 2 hours a day and as long as 20 hours a dayLittle brown bats sleep 19.9 hours per day, on average. “Large grazing herbivores” are “among the lightest sleepers, reflecting the long hours they spend feeding,” notes The Mammals That Sleep Most and Least. Horses sleep only 2.9 hours per day, donkeys sleep 3.1 hours, African elephants sleep 3.3 hours, and Asian elephants sleep 3.8 hours per day. A goat sleeps 5.3 hours per day, while a cow sleeps 4 hours per day. By contrast, a thick-tailed opossum sleeps 19.4 hours, and an Eastern American chipmunk sleeps 15.8 hours per day.