Going to Extremes
In 1938, to the glare of movie lights, Nathaniel Kleitman and Bruce Richardson emerged from a highly publicized 32-day sojourn in Mammoth Cave, Kentucky. The two researchers tried to switch to a rhythm that was four hours longer than the normal 24-hour day. If they could do so, the normal 24-hour cycle might simply be a reaction to the surrounding world. The results were inconclusive.
Two decades later, Jurgen Aschoff, a medical doctor and physiologist, expanded on Kleitman's human isolation experiments. Subjects placed in an underground bunker were allowed to turn lights on or off according to their own internal rhythms. Aschoff tracked their sleep-wake cycles, body temperature, urine output, and other physiological and behavioral variables. He concluded that humans like the plants investigated by de Mairan have endogenous circadian cycles.
In recent years, outer space has also become a popular venue for circadian research involving humans and other organisms. In an orbiting spacecraft, the normal gravitational pull felt on Earth is disrupted, which allows scientists to study the effect of lack of gravity on circadian rhythms.
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