Which of the following is true about circadian rhythms?

• A. They regulate only sleep-wake cycles and have no effect on other physiological processes.
• B. They are controlled exclusively by external environmental cues and do not persist in constant conditions.
• C. They are endogenous biological cycles that repeat approximately every 24 hours.
• D. They are unique to mammals and do not occur in plants or microorganisms.

Answer: (C)

Verified step by step guidance

1. Understand the concept of circadian rhythms: Circadian rhythms are endogenous (internally driven) biological cycles that repeat approximately every 24 hours. They are influenced by external environmental cues, such as light and temperature, but they persist even in constant conditions, demonstrating their internal regulation.
2. Clarify the scope of circadian rhythms: These rhythms regulate not only sleep-wake cycles but also other physiological processes, such as hormone secretion, body temperature, and metabolism.
3. Address the universality of circadian rhythms: Circadian rhythms are not unique to mammals. They occur in a wide range of organisms, including plants, fungi, and microorganisms, indicating their evolutionary significance.
4. Evaluate the incorrect statements: The first option is incorrect because circadian rhythms affect more than just sleep-wake cycles. The second option is incorrect because circadian rhythms persist in constant conditions, showing they are not exclusively controlled by external cues. The fourth option is incorrect because circadian rhythms are not unique to mammals.
5. Conclude with the correct statement: The correct answer is that circadian rhythms are endogenous biological cycles that repeat approximately every 24 hours, as this aligns with the scientific understanding of these processes.