Biological Clock

Internal chronometer, located in the midline of the brain above the optic nerve.  The biological clock is responsible for regulating body temperature, releasing hormones, and controlling the metabolic rate.  Its timekeeping is based on tracking the movement and interaction of molecules.  It is also the primary regulator of sleep and wakefulness.  The biological clock is what keeps us awake.  

Source:

Dement, The Promise of Sleep