What term describes external cues that align physiological functions to the solar cycle and cue the circadian rhythm?

• A. Photoreceptors
• B. Circadian rhythms
• C. Zeitgebers
• D. Suprachiasmatic nucleus

Answer: (C)

Zeitgebers are external cues that synchronize our internal biological clock with the 24-hour day. They help align physiological functions—like sleep-wake cycles, hormone release, and body temperature—to the solar cycle by signaling the brain to adjust the circadian rhythm. Light exposure is the primary zeitgeber, influencing the master clock in the suprachiasmatic nucleus. Photoreceptors detect light and convey this information to the brain, but they’re sensors, not the cues themselves. The circadian rhythm refers to the internal ~24-hour cycles, and the suprachiasmatic nucleus is the brain region that acts as the central clock. So the external cues themselves are zeitgebers.