Going to Extremes
Whatever physiological variables we measure, we usually
find that there is a maximum value at one time of day and a minimum value at
another.
— Jurgen Aschoff, "Circadian Rhythms in Man," Science, 1965.
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.
 Credit 18 |
Kleitman's and Richardson's Quarters in Mammoth Cave. Both researchers tried to adjust their sleep-wake cycles to a 28-hour cycle, with mixed results. The use of only two people and the failure to recognize that even artificial light could act as a zeitgeber resulted in unclear data. |
 Credit 8 Click to watch the video |
NASA Footage of Circadian Experiments Aboard the Space Shuttle Columbia, 1996. |
 Credit 54 |
Beetles Aloft. |