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. The SCN also governs your body temperature, hormone secretion, urine production, and changes in blood pressure. After darkness falls, the SCN signals once again and your body's level of melatonin increases, making you feel drowsy.