Nocturnal describes animals that are active at night and sleep during the day. Owls are nocturnal hunters, using their excellent night vision and hearing to catch mice in darkness. Bats are nocturnal too: they sleep upside down in caves all day, then fly out at dusk to hunt insects. Raccoons, possums, and many other creatures follow this nighttime schedule.

The opposite of nocturnal is diurnal, which describes animals active during daylight hours, like most birds, squirrels, and humans. Some animals are crepuscular, meaning they're most active at dawn and dusk, like deer or rabbits.

Why do some animals live nocturnally? Many reasons: cooler temperatures at night, fewer predators to worry about, or prey that only comes out after dark. Desert animals often become nocturnal to avoid scorching daytime heat.

You might hear people jokingly call someone nocturnal if they love staying up late and sleeping in: “My teenage brother is completely nocturnal during summer vacation.”