With sleep and circadian rhythms, it is observed in everything from plant and animals to fungi and bacteria - and of course humans. The term circadian is from the Latin circa, meaning "around" and dies / diem which means "day" - giving us a literal definition of "about a day".
It is a kind of built in clock that tells us when to be awake and when we should be sleeping. This "clock", whose scientific name is suprachiasmatic nuclei (SCN), rests in a region of the brain around where the optic nerves cross. The SCN is adjusted primarily by daylight but also by other external time cues called zeitgebers - which can be anything from the beeping of your alarm to the timing of specific meals. Light reaching this area through the retina, are turned into signals which continue their way up the optic nerve, finding its way to the SCN.
The suprachiasmatic nuclei has many functions, sending out signals to many different areas of the brain which control things like the production of melatonin and the secretion of other hormones, the production of urine, the governing of body temperature as well as changes in our blood pressure.
Scientific studies (in particular one done by Czeisler et al. at Harvard) have proven that the free-running range of a healthy adult's circadian rhythm is about 24 hours and 11 minutes (plus or minus 16 minutes). Basically, our body's clock follows the same cycle as the 24 hour rotation of the Earth.
We've already learned that light resets our biological clock or our suprachiasmatic nuclei. Light also has the ability to delay or advance our circadian rhythm depending on the timing, the type of light and the amount of light. It is not known exactly at what levels light starts to affect our sleep and circadian rhythms but some researchers believe the number to be upwards of 1000 lux.
We've learned that both light and timing play an important role between the association of sleep and circadian rhythms. Disruptions in your circadian rhythm can cause sleeping disorders as well as other issues such as general fatigue, loss of appetite, lack of alertness etc. These symptoms, which are normally associated with a lack of sleep, are referred to as circadian rhythm sleep disorders.