After describing the fundamental properties of circadian rhythms, Dr. Takahashi takes us on an exciting journey into a very special region of the brain—the suprachiasmatic nucleus (SCN). The SCN functions as a "master" biological clock that governs our physiology and certain behaviors. The clock regulates rhythms of sleep and wakefulness that make us morning larks, evening owls, or something in between. When the master clock is out of synchrony with other biological clocks in the body, symptoms of jet lag ensue.