The phrase "body clock" refers to the natural timing system that our bodies have. It helps us know when to sleep, wake up, and do other activities consistently. Full definition
The internal body clock is still alert, now trained by years and years of early mornings. (code-brew.com)
Two groups of researchers have independently discovered the long sought dual body clocks in the brain of fruit flies that separately govern bursts of morning and evening activity. (hhmi.org)
Light can throw off your kid's natural body clock and make it harder to fall asleep, so keep his room as dark as possible. (sleep.org)