Final answer:
The timekeeper of circadian rhythms is usually controlled by the biological clock, which is a tiny structure located in the suprachiasmatic nucleus (SCN) of the brain. The biological clock receives signals from light-sensitive neurons in the retina and sends signals to the body to regulate regular changes in behavior and body processes.
Step-by-step explanation:
The timekeeper of circadian rhythms is usually controlled by the biological clock. This tiny structure, located in a gland at the base of the brain called the suprachiasmatic nucleus (SCN), receives signals from light-sensitive neurons in the retina based on the amount of light present. The biological clock then sends signals to the body, causing regular changes in behavior and body processes that repeat every 24 hours.