In humans, and most [all?] other mammals, the 'Core Clock' which regulates variations in body functions is set to (approximately) 24 hours. This is the so-called Circadian Rhythm. - which in many organisms, is synchronised via daylight.
The clock is regulated via a brain area known as the Suprachiasmatic nucleus (SCN). Neurons in the SCN fire action potentials in a 24-hour rhythm. At mid-day, the firing rate reaches a maximum, and, during the night, it falls again.
Although the SCN has been extensively strudied, and the majority of its internal processes are now well understood, fundamental questions still remain. Notably, how the gene expression cycle connects to the SCN's neural firing.
“[…] whereas the core circadian oscillatory mechanism was obscure only a decade ago, we now have a standard model for the innermost clockworks. Much remains to be determined. What are the requirements for being part of the core mechanistic loop? Because cellular redox state can influence Clock:Bmal1 DNA binding and is likely influenced by clock-controlled phenomena such as electrical activity, is redox potential in or out of the loop? The cell biology of core clock components, particularly regulation of their nuclear export and entry, remains poorly understood. We also do not understand why clock mechanisms have retained (at least) two interconnected loops. What advantages does this confer over a single-loop system? As more detail of the innermost clock’s working is discerned, a major challenge to the field has become stepping back out of the loops and determining how these core oscillations are translated into a temporal program for the whole organism.”
Source: CircadianRhythms:In the Loop at Last Science 6 June 2003 Vol. 300
Also See Time Awareness