Which of the following statements about the circadian rhythm is true?

• A. It consists of the changes in a person's heartbeat throughout the stages of sleep.
• B. It is a cycle controlled by the hippocampus.
• C. It is a cycle that takes about a day to complete.
• D. It is when most dreams occur.

Answer: (C)

Verified step by step guidance

1. Understand the concept of circadian rhythm: It is a natural, internal process that regulates the sleep-wake cycle and repeats roughly every 24 hours.
2. Identify the key components of circadian rhythm: It is primarily influenced by external cues like light and temperature, and is controlled by the suprachiasmatic nucleus in the hypothalamus, not the hippocampus.
3. Evaluate each statement: The first statement about heartbeat changes is incorrect as circadian rhythm is not specifically about heartbeat changes during sleep.
4. Consider the second statement: The hippocampus is involved in memory, not in controlling circadian rhythms, making this statement incorrect.
5. Analyze the third statement: The circadian rhythm indeed takes about a day to complete, making this the correct statement. The fourth statement about dreams is incorrect as dreams are associated with REM sleep, not directly with circadian rhythm.