noun
biological clock; body clock
Refers to the internal mechanism that regulates circadian rhythms and physiological cycles.
体内時計が狂うと、睡眠のリズムが乱れる。
When your biological clock is disrupted, your sleep rhythm gets thrown off.
海外旅行の後は、体内
After traveling abroad, it takes a few days to adjust your body clock to the local time.