An excerpt from Catharsis: On the Art of Medicine by Andrzej Szczeklik, from the University of Chicago Press website:
It is not just the world that sends its rhythms coursing through us. There are also rhythms inside us. There are so many rhythmic processes happening in our bodies, from the obvious ones, like sleeping and waking, to the most well hidden, like the secretion of hormones into the blood, that to explain their uncanny regularity and synchronicity we have adopted the figurative idea of the biological clock. Long before it was discovered, everyone agreed that if this extraordinary chronometer really did exist, then every last cell of our bodies would be able to tell the time from it.
Nowadays we locate it in the brain, in the part called the hypothalamus. The biological clock runs in two concentrations of gray matter, known as the hypothalamic nuclei, and so does its most essential part—the circadian oscillator. The clock’s mechanism appears to be determined by a cycle of recurring reactions: the transcription of genes and the synthesis of proteins. These reactions form a feedback loop: so-called clock genes code proteins, which accumulate and retroactively obstruct the transcription of genes. As protein disintegrates, transcription gets going again, and the protein production cycle is resumed. This “clockwork” system, characterized by rhythmicality, is common to all species, from the fruit fly to man. It is teamed with the emission of circadian signals, which depend on changes in the cell’s membrane potential. Once in existence, they spread into the nearest vicinity and to other areas of the brain as well.
But what use would a watch be if you couldn’t set it to local time?