Dogs are day animals. That may be the secret to why they’ve become such popular pets. Scientists at the University of Michigan looked at hundreds of years’ worth of data about wild and domesticated dogs’ DNA. What they found was that pet dogs had a key difference: a gene that indicates their desire to stay awake during the day, as opposed to being nocturnal like wolves. Basically, this means that dogs throughout the years have evolved to stay awake with their pack—in this case, their owners.