Routine changes in your behavioral, mental, and physical functions that occur over the course of a day are regulated by your 'biological clock.' This tiny area of the brain, the suprachiasmatic nucleus (SCN20,000 neurons. When light enters your eye, it activates neurons in the retina that convert photons (light particles) to electrical signals. These signals travel along the optic nerve to the SCN which in turn stimulates several brain regions, including the pineal gland. The pineal gland responds by switching off production of the hormone melatonin, and this makes you feel more awake. After darkness falls, your biological clock or suprachiasmatic nucleus (SCN) signals your pineal gland again and your body's level of melatonin increases, making you feel drowsy. More tomorrow.