A lantern hisses on a wooden table stacked with ticking instruments.
Two cots sit against damp limestone, blankets rumpled, a wash basin on a stand.
No window, no sunrise, no church bell to mark the hour.
Just a man pressing a thermometer to his own skin every couple of hours, whether he is asleep or not.
That routine was the only clock that mattered to the two men living inside it.
What their bodies did with the missing clock is the question the first answer has to settle.
What a cave can do that a bedroom cannot
A cave deep enough strips away the signals a body normally uses to set itself: daylight, the swing between warm afternoon and cool night, the noise of other people keeping ordinary hours. Physiologist Nathaniel Kleitman suspected that the human rhythm of sleep and body temperature is generated inside, then nudged into line each day by those outside cues.
Take the cues away and whatever rhythm survives has to belong to the body itself. Body temperature is the readout that makes this visible, since it follows a daily curve, lowest in the early morning and highest in the early afternoon for people who sleep at night.
A man can convince himself he feels alert at the wrong hour. His temperature trace is harder to argue with. So Kleitman and his graduate student Bruce Richardson set out to force their bodies onto a day no calendar recognizes, a 28 hour cycle instead of 24. That meant only six of their long days fit into a calendar week, and a lantern told them when their day began and ended, nothing else did.
Two beds, one chamber, two different bodies
The setting was almost homely for a scientific first. The men brought lanterns to regulate their light exposure, a table and a bunk bed, and staked out a rock chamber about 26 feet high by 65 feet wide in Mammoth Cave in Kentucky, well over a hundred feet below the surface. They settled in Rafinesque Hall, a large passageway near the cave’s Historic Entrance.
Comfort was not part of the deal. The cave’s constant humidity and average temperature of around 54 degrees Fahrenheit meant the researchers wore multiple layers of clothing and heavy coats to stay warm. And they depended on people above for food, drinking water and contact with the outside world, since a waiter from the Mammoth Cave Hotel carried down lunch and mail.
Of their underground residence, Richardson remarked, “I guess we missed hot baths more than anything else.” The two kept a regular routine of eating, sleeping, reading, writing and walking. The cave had other residents too, and Kleitman had to devise traps to stop rats from climbing into bed with the sleepers.
The number that split two bodies apart
From June 4 to July 6, 1938, the pair camped out in the cave to study the body’s ability to conform to a non 24 hour cycle. Each “day” they slept for 9 hours, worked for 10 and rested for another 9.
After 32 days underground, the two results refused to agree with each other. Richardson, a man in his twenties, was able to adjust to the new sleep pattern, while the older Kleitman had difficulty.
Kleitman, then 43, found that his body clock did not move toward a non 24 hour schedule. He would get tired at 10 at night and feel awake again eight hours later. Same cave, same lanterns, same alarm, yet completely different internal clocks emerged from identical conditions.
What a sample of two could and could not prove
That split was the finding, and it came with an obvious limit. Because only two subjects were used, Kleitman did not publish the findings immediately, and later wrote a book about early sleep research instead.
Even so, contemporary reporting drew a careful conclusion. Breaking away from the 24 hour rhythm was not impossible, but the ability varied between individuals in ways the two men could not explain at the time.
Something underneath both men stayed stubborn all the same. Under the uniform temperature, illumination and calm of the cave, their experiment indicated that the human body roughly maintains a 24 hour temperature cycle even without external cues. You can read the whole episode in the park service account of the study.
Why this still matters far above any cave
Modern instruments have since pinned down the number the cave could only hint at. Careful laboratory work found that the intrinsic period of the human circadian pacemaker averages 24.18 hours, about 24 hours and 11 minutes, in both younger and older adults. That is a tighter 24 hour rhythm than the roughly 25 hour figure researchers once assumed.
The correction matters for anyone planning a trip off this planet. When the human clock was thought to run near 25 hours, it seemed it would synchronize easily to the 24.65 hour Martian sol. But the finding that the intrinsic period sits so close to 24 hours led to the demonstration that the pacemaker cannot entrain to such a day, which is close to the stretched day the two men tried to impose on themselves with lanterns and an alarm clock.
The question has not faded. Scientists chasing it now run cave isolation studies and sealed chamber experiments with far more sensors than one thermometer, and a four person crew is living inside NASA’s simulated Mars habitat in Houston until the end of October. What holds up from the cave is the smaller, harder point, that the clock inside a human body does not simply obey the schedule it is handed.
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