Which factor acts as a zeitgeber to reset circadian rhythms?

Which factor acts as a zeitgeber to reset circadian rhythms?

• A. Social cues
• B. Light
• C. Temperature fluctuations
• D. Food availability

Answer: (B)

Light is the strongest cue that resets circadian rhythms because it provides a reliable signal of day and night that the brain uses to synchronize the internal clock. Specialized light-detecting cells in the retina send information about dawn and dusk to the brain’s master clock, the suprachiasmatic nucleus. This input adjusts the timing of clock gene expression throughout the body, aligning activity and sleep with the 24-hour day. Exposing the organism to light in the morning tends to advance the clock, while light exposure at night tends to delay it, helping maintain a regular daily rhythm. Other factors can influence rhythms in particular contexts, but they’re not as universal or robust as light. Social cues, temperature changes, and feeding times can shift timing in some species or tissues, but light remains the primary cue that entrains the central clock to the environment.

Light is the strongest cue that resets circadian rhythms because it provides a reliable signal of day and night that the brain uses to synchronize the internal clock. Specialized light-detecting cells in the retina send information about dawn and dusk to the brain’s master clock, the suprachiasmatic nucleus. This input adjusts the timing of clock gene expression throughout the body, aligning activity and sleep with the 24-hour day. Exposing the organism to light in the morning tends to advance the clock, while light exposure at night tends to delay it, helping maintain a regular daily rhythm.

Other factors can influence rhythms in particular contexts, but they’re not as universal or robust as light. Social cues, temperature changes, and feeding times can shift timing in some species or tissues, but light remains the primary cue that entrains the central clock to the environment.