Thomas A. Wehr
Frederick K. Goodwin
Our body is like a clock; if one wheel be amiss, all the rest are disordered, the whole fabric suffers: with such admirable art and harmony is a man composed.
The Anatomy of Melancholy (1628)
RHYTHMS IN NATURE—the alternation of day and night, the tides and the seasons—govern our lives and structure our experience. The degree to which nature's cycles influence culture is obvious. Less obvious is the fact that they are also impressed upon our genes, for we generate within ourselves days, months, and seasons that mirror and anticipate the rhythmic changes around us. Our biological rhythms make each of us a microcosm of the geophysical world.
Unlike the motions of the planets, biological clocks are imprecise, and their synchronization with external rhythms depends upon their being continually reset. For this purpose we possess special sense organs that lock onto external time cues such as the rising and setting of the sun and that make corresponding adjustments. As we live out our lives, our biological self is always tuned to the rhythms of the world around us, and we are forced to keep time with its march.
Our internal rhythms constitute a kind of temporal anatomy. Each day our body's temperature rises from a predawn nadir to its evening peak. At night the pineal gland secretes a hormone, melatonin. When we fall asleep, growth hormone briefly appears. The adrenals emerge from quiescence abruptly in the middle of sleep and are most active at dawn. And nearly every function of the organism exhibits a 24-hour, or circadian, pattern of variation with its own characteristic timing and waveform. The classical principle of homeostasis must be amended to encompass such physiological variation: cyclic