Why We Need Sleep

Lions can snooze for 15 or 20 hours a day, while giraffes can go for weeks without getting even a five-minute nap. Cheetahs doze for 12 hours a day, while horses get only two or three hours of shut-eye.

Diet and environment determine how much sleep various mammal species require, says Jerome Siegel, a researcher at the University of California's Neuropsychiatric Institute who analyzed the sleeping habits of 60 mammals and found a distinct pattern. Meat eaters get more sleep than grazers, and omnivores fall somewhere in between.