Two men carried a bunk bed, a table and a stack of lanterns into a limestone room with no windows at all.
It sat deep under the Kentucky hills.
Outside, June turned into July.
Inside, the men stopped keeping track of either.
They were trying to convince their own bodies that a day lasted 28 hours instead of 24.
Only one of them ever came close.
What actually bends a body clock, and what refuses to move
The scientist behind the plan was a University of Chicago physiologist who had built a career on sleep, and who believed the daily cycles of temperature and sleep were dictated by patterns of habit and behavior rather than rhythms intrinsic to the brain or body. If that rhythm was only a social convention, a person kept away from clocks, daylight and noise should be able to retrain it onto any schedule he liked.
But the measurements pointed the other way. Tracking the daily rhythm of body temperature, the researchers found an endogenously generated 24 hour cycle, despite the self imposed 28 hour schedule. That result is the seed of what chronobiology now takes for granted, a clock built inside the body and only nudged into line by daylight.
So the two men did not land in the same place. Richardson, 20 at the time, felt he was better able to adjust, while Kleitman, then 43, found his body clock would not budge, getting tired at 10 at night and alert again eight hours later.
A rock room built for silence, not for comfort
The scene looked almost domestic for a cave. The men brought lanterns to regulate their own light exposure, along with a table and a bunk bed, staking out a spot in a chamber about 26 feet high by 65 feet wide. The park service identifies the place as Rafinesque Hall, a large passageway near the Historic Entrance.
And the setting did the real work. The cave, 140 feet underground with no natural light, gave them a location free of visual cues of day or night, holding steady at a chilly 54 degrees Fahrenheit. No clock face, no window, no shift in warmth to betray the hour.
Food came down from the surface on the men’s own timetable. Kleitman gave strict instructions to the nearby hotel for two meals a day, timed to the pair’s drifting schedule rather than the clock outside. The isolation was never solitude, more a carefully managed bubble cut off from cues a body leans on without noticing it does.
Thirty two days, and the hard numbers behind the split
The men did not improvise the schedule once underground. Each stretched day they slept for 9 hours, worked for 10, and rested for another 9, with an alarm clock keeping them to the experimental 28 hour cycle. Because the cycle ran longer than a real day, the calendar outside and the clock inside drifted apart fast.
From June 4 to July 6, 1938, Nathaniel Kleitman and his student Bruce Richardson camped out in Mammoth Cave to study the body’s ability to conform to a non 24 hour cycle. The stay ran 32 days, which works out to roughly 27 of their own stretched days below ground.
Richardson felt he was adjusting after just a week. Oral temperature readings taken through each long day gave the pair their hardest evidence, and it was that trace, rather than how the men felt, that carried the finding. Kleitman’s temperature stuck to the 24 hour cycle, and he grew more irritable and exhausted with each altered day.
Where the experiment ran into its limits
With only two subjects, nobody could say whether age explained the split or whether two nervous systems simply behaved in their own ways. Because the sample was so small, Kleitman did not publish the findings right away, setting the work down later in his 1939 book on sleep. A sample of two can point toward an idea, but it cannot settle one.
Even so, later researchers took that uncertainty into other holes in the ground. The line of field experiments that began at Mammoth Cave ran on into the 1960s, ending with British physiologist John Mills in Stump Cross Caverns in the Yorkshire Dales. Stripped of outside cues, human rhythms tend to free run a little longer than 24 hours rather than stretching to whatever schedule is forced on them.
That cave result was an opening question, not a final answer. Other self experiments have since pushed the body under stranger pressure still, including this centrifuge arm experiment, where a volunteer’s physiology met conditions it never evolved to handle.
What that one cave still tells modern science
Modern chronobiology, the field partly built on that summer underground, now treats the internal clock as a basic feature of human physiology. Jet lag, shift work and irregular sleep all run into the same stubborn rhythm the two men kept bumping into nearly a century ago.
Their approach became a template too. Later isolation work, including studies run inside a simulated space station, used the same basic move, stripping away light, noise and clocks, then watching what the body does when nobody tells it what time it is.
The chamber is still there. Rafinesque Hall sits on park tour routes a short walk from the historic entrance, and researchers from the University of Chicago returned in 2024 for a visit marking the original study.
The honest caveat is that two men cannot represent everyone. Yet the stubborn 24 hour rhythm they ran into keeps turning up in sleep laboratories, traced back to a small, damp, lantern lit room that reshaped how science understands a day.
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