The day–night cycle that occurs approximately every 24 hours is the _____.

• A. ultradian rhythm
• B. cell cycle
• C. mitotic cycle
• D. circadian rhythm

Answer: (D)

Verified step by step guidance

1. Understand the concept of biological rhythms: Biological rhythms are recurring cycles in biological systems that help organisms adapt to environmental changes. These rhythms can vary in duration and are classified into ultradian, circadian, and infradian rhythms.
2. Define circadian rhythm: A circadian rhythm is a biological cycle that occurs approximately every 24 hours. It is driven by an internal biological clock and is influenced by external cues such as light and temperature. Examples include the sleep-wake cycle and hormonal fluctuations.
3. Differentiate between ultradian, circadian, and infradian rhythms: Ultradian rhythms occur more frequently than once a day (e.g., heartbeats), circadian rhythms occur roughly every 24 hours, and infradian rhythms occur less frequently than once a day (e.g., menstrual cycles).
4. Eliminate incorrect options: The cell cycle and mitotic cycle refer to processes of cell division and are not related to daily biological rhythms. Ultradian rhythms occur more frequently than 24 hours, so they are not the correct answer either.
5. Conclude that the correct answer is circadian rhythm, as it matches the description of a biological cycle occurring approximately every 24 hours, such as the day-night cycle.