You love to say you're a night owl, yet your body is telling a very different story. Your circadian rhythms, the routine changes in your behavioral, mental and physical functions that occur over the course of a day, are regulated by a tiny area of the brain commonly known as your 'biological clock.' The suprachiasmatic nucleus (SCN) is shaped like a pine cone, but is about the size of a grain of rice, though it contains about 20,000 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. 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.
This intricate and complex biological system responds daily and automatically to the rhythms of day and night. Like all biologic beings, you respond to your environment. You are a part of the whole. When you impose an artificial tempo to your day by going to bed too late, there will be real health consequences.