Circadian rhythm is a cycle that tells our bodies when to sleep, as well as when to wake and eat.

• A. Circadian Rhythm
• B. Metabolic Cycle
• C. Endocrine Cycle
• D. Ultradian Rhythm

Answer: (A)

The main idea here is the body's 24-hour internal clock that governs when we sleep, wake, and eat. This daily timing system is called the circadian rhythm. It’s driven by the brain’s internal clock and synchronized to the day–night cycle through light signals, helping regulate sleepiness, alertness, and when we tend to eat. The other terms describe different kinds of cycles: ultradian rhythms are shorter than a day (like the ~90-minute sleep cycles), and metabolic or endocrine cycles aren’t the standard labels for the daily sleep–wake–eat pattern. So the circadian rhythm best fits the description.