Explain the circadian rhythm.?

Circadian rhythm. Circadian rhythm refers to a natural physiologic cycle in which certain processes ebb and flow in a predictable pattern over the day. In humans the basic cycle is tied to the sun with sleeping at night and activity during the day. But there are other cycles. For example, your lung airways naturally get tighter at night (4 am) and that is why asthma patients often get their first symptoms at night.