In humans our biological clock drives our circadian rhythms. Humans actually have many biological clocks within us that are made up of groupings of interacting molecules in cells throughout our bodies. There is one large master clock in the brain that coordinates all the other clocks and keeps them in synch with each other.
The circadian rhythms within our bodies work to tell us hen it is appropriate to be asleep or awake as well as other things like when we should be hungry or when our bodies need a rest after working hard. These clocks are incredibly intricate. Humans are not the only ones that have them either ever organism has some kind of body clocks.