Circadian rhythm disorders are disruptions in a person's circadian rhythm -- a name given to the "internal body clock" that regulates the (approximately) 24-hour cycle of biological processes. The term circadian comes from Latin words that literally mean around the day. There are patterns of brain wave activity, hormone production, cell regeneration, and other biological activities linked to this 24-hour cycle.
The circadian rhythm is important in determining sleeping patterns such as when we sleep and when we wake, every 24 hours.The normal circadian clock is set by the light-dark cycle over 24 hours.