circadian rhythms (noun) 

Circadian (sur-KAY-dee-un) rhythms are natural, internal processes that regulate the body’s 24-hour sleep/wake cycle. They primarily respond to light and dark. As I used it today: “Yes, there are ‘night people’ –  individuals whose circadian rhythms favor waking late and working at night. I’m one of them. But I don’t make the mistake of pretending that I am better off submitting to my natural inclinations.”