Where is the main circadian clock located in mammals?

• A. Pineal gland
• B. Hippocampus
• C. Suprachiasmatic nucleus
• D. Cerebellum

Answer: (C)

The master clock in mammals is located in the suprachiasmatic nucleus, a small group of neurons in the anterior hypothalamus just above the optic chiasm. These neurons generate intrinsic ~24-hour rhythms through cellular clock gene feedback loops and keep the body's daily timing in sync with the environment by receiving light input from the retina via the retinohypothalamic tract. This clock coordinates downstream rhythms across the brain and body and communicates with the pineal gland to regulate melatonin production, signaling night. While the pineal gland helps implement the nightly signal, it is not the master clock itself. The hippocampus and cerebellum serve other brain functions like memory and motor control, not circadian timing.