Circadian rhythm is a biological process that follows a roughly 24-hour cycle and is found in many living organisms, including humans. It is often referred to as the body's internal CLOCK and is regulated by light and darkness. The term 'circadian rhythm' comes from the Latin words 'CIRCA' meaning 'AROUND' and 'diem' meaning 'DAY'. The crossword solution for this term is 'CLOCK', which is an instrument used to measure and keep track of time.