Have you ever taken a long-haul flight? If so, you know that the timepiece in your head sometimes ignores the one on your wrist. If you leave Boston in the evening and, seven hours later, arrive in Paris at breakfast time, your body screams, "Why am I getting up? It's the middle of the night!" Croissants or no, your internal clock persists in its own rhythm, and it can take several days to synchronize your sleeping patterns with your new surroundings.
In fact, this powerful clock is very small. It lies within the suprachiasmatic nucleus (SCN)--an area of the brain no larger than a grain of rice--which sits in the hypothalamus directly above the optic chiasma (where the right and left optic nerves meet). The SCN takes cues from light receptors in the retina to send its own signals to the pineal gland, which releases various hormones in response. In this way, the SCN orchestrates our circadian rhythms, pacing all sorts of daily physiological fluctuations, including body temperature, blood pressure, heart rate, hormone levels and sleep-waking times.