Anyone who has ever flown east or west at 500 knots for more than a few hours has experienced firsthand what happens when the body's internal clock does not match the time zone in which it finds itself. Up to a week may be needed to get over the resulting jet lag—depending on whether the master clock, which is located deep inside the brain, needs to be advanced or slowed to synchronize when the body and brain want to sleep with when it is dark outside. Over the past several years, however, scientists have learned, much to our surprise, that, in addition to the master clock in the brain, the body depends on multiple regional clocks located in the liver, pancreas and other organs, as well as in the body's fatty tissue. If any one of these peripheral clocks runs out of sync with the master clock, the disarray can set the stage for obesity, diabetes, depression or other complex disorders.