There’s a reason why people who stay up late are called night owls. That’s because owls are always awake and active when the sun is down. This is called nocturnal behavior, and it’s common among many animals. They become more active at night to hunt, mate, or avoid heat and predators.

Nocturnal animals have evolved physical traits that let them roam in the dark more effectively. The eyes get bigger and the pupils widen. Owl eyes, for example, are so big that they can’t move in the socket, but their wide pupils help them collect more light.

A reflective layer called tapetum sits behind the retina; any light that passes through the eye reflects back onto the