Your Body is a Time Machine.

Your body clock is a tiny cluster of nerve cells in the center of your brain, about the size and shape of a letter "V" on this page. Located just above a major junction for nerves coming from the eyes, the clock relies on sunlight to keep you synchronized with planetary time.

Like the thermostat connected to your home furnace, your body clock makes your body temperature and blood pressure rise before you usually awaken in the morning. It triggers the release of "get up and go" hormones you need to start the day. Your body clock makes you predictably alert in the daytime, sleepy at night, and hungry when you habitually eat. It controls cell division, bone growth, and other basic body maintenance. People who can't utilize sunlight cues--those who are totally blind--commonly experience sleep problems and other rhythm disruptions.

While daily rhythms are most prominent in our lives, shorter and longer ones affect us, too. Many illnesses in women, for example, worsen just before monthly menstruation. In the United States and other countries of similar latitude, more babies are born in late summer than at any other time of year. The birth rates reflect seasonal variations in sexual activity, most likely spurred by heightened secretion of the male sex hormone testosterone in late autumn when days grow shorter.

The tick and tock of your body clock holds numerous implications for your health. When you are well, body rhythms work in synchrony, like a finely-tuned orchestra. Illness, travel across multiple time zones, and shift work schedules often throw body rhythms out of tune.