How does your body know when it is time to go to sleep and to wake up? Inyour brain, in a place called the suprachiasmatic nucleus (SCN), there is a col-lection of cells that have the ability to keep time and function as a pacemakerfor your sleep-wake cycle. It turns out that this pacemaker controls not onlythe times when you are sleepy or alert, but it also controls the function ofmany systems in the body. The word many, in fact, may be an understate-ment. Most of the systems in the body have a pattern that varies over a twenty-four-hour period. This is true for the secretion of many hormones, bloodpressure, heart rate, and other functions in the body. This natural, internalrhythm in function has been called the circadian rhythm. The word comesfrom circa meaning “about” and diem meaning “day.” In other words, the circadian rhythm changes the way many systems in the body work over thetwenty-four-hour day so that the function of the systems matches what thebody needs. As a result, we usually don’t have to go to the bathroom and wedon’t have hunger at night. If you have traveled across time zones you knowhow discombobulated or out of sync you can feel because of a disconnectbetween your own body clock and the time where you happen to be.