For humans, sleep is a necessity, a mystery, and a luxury. It’s not known why we need it but we do—and an hour more or less of it can make your day either great or grumpy.

Most animals sleep, too, says Jerome Siegel, a psychiatrist at the University of California, Los Angeles, but in ways that are just about as varied as the animal kingdom itself. These variations include duration and depth of sleep, and even how it works in the brain.

From dogs that doze off and on all day to dolphins that sleep using only half their brains, here’s a look at the many different ways that animals sleep.

Humans, like all other great apes, are monophasic sleepers, meaning