Final answer:
A circadian cycle is about 24 hours long and is controlled by a biological clock located in the brain.
Step-by-step explanation:
A circadian cycle is about 24 hours long. The term 'circadian' comes from the Latin words 'circa' meaning 'around' and 'diem' meaning 'day'. It refers to the internal rhythm or biological cycle that repeats approximately every 24 hours. This cycle is found in most eukaryotic organisms and is controlled by a biological clock located in the brain. The circadian rhythm helps regulate various biological processes, including sleep-wake cycles, hormone production, and body temperature fluctuations.