Why do flies fly in circles around the centre of the room?

Theres always one every day, all through summer. There's no logial reason though, ive heard the one about tey fly around heat sourse and lights, but it's the middle of the day and the light hasnt been on since last night. And the fan thing, but we dont have a ceiling fan in the room. What attracts them to fly in a little circle under the ceiling light?