Sunlight, cortisol and melatonin. I like this rhythm because of the integral interaction between humans and the sun. It all starts in a teensy weensy spot right in the middle of the brain (halfway between the eyeballs and the back of the head) which contains a small bundle of nerves called the suprachiasmatic nucleus (SCM) within the hypothalamus, which is the traffic control officer for many of the brain's nerve signals.
The SCM receives light and dark signals from the presence and absence of sunlight. From this information it tells the body what time it is and thus what it should be doing. A fascinating thing about the SCM is that although it receives the light signals through the eyes it is still fully active and functional in those who are otherwise completely blind (also, it's present and functioning in sightless animal species). It's that primitive and important of a function, even more so than sight itself.
When the SCM sees light, it says, "suns up! time to get moving!" The form of its signaling is called the morning cortisol spike. Cortisol is most commonly known as a stress hormone, one which floods the body during those "fight or flight" body responses (previously helpful for escaping hungry predators, now used mainly in rush hour traffic). This amping up of the system is a primary factor in getting someone up and moving in the morning.
Throughout the day, cortisol slowly climbs back down to its low pre-dawn level, waiting for the next day's first morning burst of sunlight to start all over again.
A fascinating aspect of melatonin is that it spikes at night for all animals. Nocturnal animals which are most active at night appear to receive a similar effect from a nightly burst of melatonin as the average person does his or her morning burst of cortisol. Each respective species has their own "wake up" and "go to sleep" hormonal response.
Elegant, responsive, and endlessly adapting to signals both within and beyond our bodies, bodily rhythms are necessary, complex and deserve a little recognition every once in a while.