Your Body clock.
Circadian translates to roughly (circa) day (dian)
The are a biological version of a clock. The rhythms respond to daylight, which regulates the clock in order to determine when the body is able to repair cells and sort out memory.
This internal ‘clock’ is present not just in humans but microscopic organisms also – like bacteria. Keeping this clock working well means that you can get off to a flying start in the morning for example. It will mean that your are operating efficiently and not coping with a constant feeling of jet lag, which can happen when your sleeping patterns are confused.
A great reference to circadian rhythm can be found on the BBC In our time podcast: circadian rhythms.
More info to follow.