Diurnal describes creatures that are active during the daytime and sleep at night, just like most humans do. If you notice squirrels scampering up trees in the afternoon or robins hopping across your lawn in the morning, you're watching diurnal animals going about their daily business. These animals have evolved to thrive in daylight: they hunt, gather food, and socialize when the sun is up.

The opposite of diurnal is nocturnal, which describes animals like owls and bats that are active at night. Think of it this way: diurnal animals are on the same schedule as a typical school day, while nocturnal animals are just waking up when you're going to bed.

Scientists use this word because an animal's activity pattern affects everything about how it lives. Diurnal animals often have excellent color vision for spotting food and predators in daylight, while nocturnal animals might rely more on hearing or smell. Most primates, including humans, are diurnal: our eyes work best in bright light, and we naturally feel sleepy when it gets dark.

The word can also describe plants that open their flowers during the day, or anything else that follows a daily cycle tied to daylight hours.