This is a podcast from the archives of the BBC Reith Lectures.

This lecture in the series Mechanics of the Mind, given by Colin Blakemore, was originally broadcast in 1976.

Nathaniel Clytemann, a Russian emigre who settled in the United States in 1915,

and Bruce Richardson, an American colleague, spent 32 days alone in the depths of

Mammoth Cave, Kentucky in 1938.

Under the artificial lights in their underground home preserved the rhythmic cycles of the day.

But the day by which Clytman and Richardson lived was not the day of this earth.

They forced themselves to join the routine of an unknown planet, one with a 28-hour day,

19 hours for waking and 9 for sleep.

Could their bodies tolerate this sudden spaceflight to a world of different time?

Clytman, a pioneer of the scientific study of sleep at the University of Chicago,

had already developed methods of measuring continually the normal fluctuations of body temperature,

hotter in the early afternoon, cooler in the early morning,

and of recording the tossing and turning of

restless sleep. He used the same techniques in Mammoth Cave to see whether his temperature

cycle and sleep pattern and those of Richardson, his companion, would stick to Earth time or switch

to the clock of Planet X. To be brief, Clytman stuck and Richardson switched.

Clytman was unable to adjust to the 28-hour day. His body temperature stubbornly followed the

rhythm of the world outside the cave, and his sleep was inadequate and fitful, especially

when every six days, midnight in the cave, coincided

with noon in the real Kentucky. Richardson, on the other hand, adapted well in his temperature

cycle and his sleep pattern. Similar experiments in underground bunkers in Germany and in the continuous daylight of summer in the Arctic Circle have proved that man has a clock in his brain, a clock set to the time cycle of his natural home.

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