Which structure sends circadian signals to regulate bodily functions?

• A. The Hypothalamus
• B. The Pineal Gland
• C. The Amygdala
• D. The Suprachiasmatic Nucleus

Answer: (D)

Understanding circadian organization starts with the brain clock. The suprachiasmatic nucleus sits in the hypothalamus as the master clock that orchestrates daily rhythms across the body. It receives daylight information from the retina via the retinohypothalamic pathway and uses that input to set the internal timing. From there, it sends timing signals to other brain regions and to the pineal gland. The pineal gland then adjusts melatonin release to reinforce the sleep–wake cycle, but it does not generate the circadian signal on its own; it acts downstream of the master clock. The hypothalamus is the broader region that houses the SCN, while the amygdala is more involved in emotion. So the structure that directly sends circadian signals to regulate bodily functions is the suprachiasmatic nucleus.