Circadian is a science-and-medicine word for things that follow a roughly 24-hour pattern — about one day. We use it most about the body: when you feel sleepy, when you feel hungry, when your temperature rises and falls, when some hormones go up and down. Doctors talk about your circadian rhythm, meaning that inner day-clock. It is not a clock you can see; it is a pattern your body keeps, even in a dark room, though actually light (especially morning light) is what keeps it lined up with the real day. Jet lag is what happens when you fly far and the sun’s day no longer matches your body’s day. The word is fairly formal. Ordinary people more often say body clock. You will meet it in biology class, sleep ads, and hospital leaflets, not much in chat. It does not mean “circular” or “going in a circle,” even though it looks a bit like those words. It means “about a day.”