We humans may need our eight hours of sleep a night to keep our brains functioning, but that’s not the only duration, or reason, why animals sleep. Bats sleep for 20 hours out of the day while giraffes snooze for less than two hours a day. And while humans completely conk out, oblivious to the world, other animals like some whales and seals, shut off only portions of their brain at a time, keeping half their mind at work at all times.   Why is there such variety in the animal kingdom? It’s something scientists have been pondering for a long time.