What is the brain region commonly abbreviated as SCN that coordinates the sleep-wake cycle?

• A. Suprachiasmatic Nucleus
• B. Hypothalamic Nucleus
• C. Pineal Gland
• D. Amygdala

Answer: (A)

The master clock that coordinates the sleep-wake cycle is the Suprachiasmatic Nucleus, located in the hypothalamus just above the optic chiasm. It keeps our internal time by receiving light information from the eyes through specialized retinal cells, via the retinohypothalamic tract. That light signal tells the SCN what time of day it is, so it can align our body’s rhythms to a roughly 24-hour cycle. The SCN then coordinates signals to downstream systems, including the pineal gland, which releases melatonin in the dark to promote sleep and reduces melatonin when light is present to promote wakefulness. The pineal gland itself isn’t the clock; it’s an output that follows the SCN’s instruction. Regions like the amygdala aren’t about timing the sleep-wake cycle, and a generic “hypothalamic nucleus” label isn’t precise enough to designate the master clock.