Longman Dictionary of Contemporary English
body clock
noun
EXAMPLES FROM CORPUS
▪ Adjusting your body clock Sometimes we want to change the timing of our life-style.
▪ Clearly, a rhythm in food intake might be able to adjust the body clock via several mechanisms.
▪ In all cases, the effect of the body clock is being accentuated by our life-style, generally by means of the hormone adrenalin.
▪ Is it a function of our body clock or our life-style, or due to some interaction between the two?
▪ The need for time-cues to produce a body clock with a stable 24-hour period has already been described.
▪ This result might indicate that the body clock is slow to mature in premature babies.
▪ This theory also implies that the internal body clock takes up to five years to develop fully.
▪ Those few studies that have been carried out in free-running experiments suggest that the body clock might run slightly faster.
Wiktionary
body clock
n. biological clock