body clock 
noun
A person’s or animal’s biological clock: jet lag is the consequence of a mismatch between our body clock and the external timing...
- Our body clock, our internal clock, is linked to the 24-hour day.
- Melatonin resets the body clock to synchronize metabolic functions with times of activity and rest.
- Jet lag occurs, I learned, because changes in time zones confuse our internal body clock or our ‘circadian rhythms’; profound biorhythms which date back to Stone Age man.