The pineal gland is sensitive to light.

• A. False
• B. True
• C. Only during infancy
• D. Only in nocturnal animals

Answer: (A)

Light doesn’t directly activate the pineal gland. In humans, light is detected by the retina, and that information travels to the brain’s master clock, the suprachiasmatic nucleus. The SCN then controls the pineal gland’s activity through sympathetic nerve connections. This neural signaling adjusts the enzymes that make melatonin, so melatonin production increases in darkness and drops with light. So the gland’s behavior tracks the light-dark cycle, but the gland itself isn’t directly light-sensitive. In some other animals, the pineal gland can be directly light-sensitive, but in humans it is regulated by neural signals rather than by light shining on the gland itself.