When University of Chicago physiologist Nathaniel Kleitman told fellow faculty members he was seeking a locale for a month-long sleep experiment — someplace as isolated from the rhythms of day and night as the Arctic in summer — a colleague in the geology department said he knew just the spot.
Between 10 million and 15 million years ago, in what is now south‑central Kentucky, trickles of groundwater began probing the cracks in a fossil seabed. Over the eons, the pockets grew and grew until they’d formed the most extensive cave system in North America — over 400 miles of underground chambers, canyons, tubes, shafts, and passageways, interwoven with Stygian rivers.
Within a few months of the suggestion, on June 4, 1938, Kleitman and a companion — a graduate student named Bruce Richardson — made the short hike to their Mammoth Cave camp, 140 feet beneath Earth’s surface. Unlike the era’s other trailblazing voyages (Lindbergh leaping the Atlantic, Byrd bagging the North Pole), this one would go inward: not just below terra firma but deep into the mysteries of the human brain and body. The pair would spend 32 days and nights keeping to a 28‑hour sleep‑wake schedule, with their results having profound implications for the nascent science of sleep.
Before Kleitman’s expedition, only a handful of scientists studied sleep — and not a single one did so full‑time. Most saw slumber as a nonevent, a nightly state of suspended animation. Many considered it a vestige of humanity’s primitive past, which could be minimized or eliminated altogether.
Today, the quest to understand sleep — and to apply that understanding to our daily lives — has become a global obsession. Sleep research centers can be found at every major university. Over 2,500 sleep clinics operate across America, and 4,000 more in other countries. The
World Sleep Society, which represents scientists and health care professionals on every continent but Antarctica, boasts 14,000 members. Over the past 90‑odd years, sleep has gone from an afterthought to a central element in our notions of well‑being.
How sleep science evolved to its current state of sophistication is thanks to the efforts of early sleep explorers, like Kleitman, who spent decades working to solve puzzles whose importance few others could grasp. When they hit dead ends, they analyzed their errors and set off in new directions. They risked futility and failure to advance a field that was widely regarded as scientifically irrelevant. In the process, they changed our nights — and days — forever.
In the years before entering Mammoth Cave, Kleitman had set himself one central task: to transform sleep science from an obscure backwater into a thriving discipline. In the early 1930s, he had launched a series of studies aimed at scrutinizing sleep with a granularity no one had yet attempted. He had intertwined goals: to analyze the rhythms of sleep and test how different variables in waking behavior affected sleep rhythms. By charting factors such as body temperature, body movement, and mental acuity at different times of day and night, and under varying conditions and routines, he hoped to map not only the contours of normal sleep but also to learn what distinguished it from abnormal varieties.
Of all the variables Kleitman aimed to examine, only body temperature had been studied extensively. It had long been known that temperatures in humans fluctuated by a degree or two each day, reaching a minimum sometime after midnight and a maximum in the afternoon. Kleitman hypothesized that the daily cycles of both temperature and sleep were dictated by patterns of habit and behavior (such as food intake, muscular exertion, and brain activity) rather than rhythms intrinsic to the brain or body. He also posited that both cycles could be changed without harm, though a person might need some time to adjust.
For a month, Kleitman tried to shift his own sleep‑wake rhythm to follow a 48‑hour cycle, rather than the traditional 24-hour one. His temperature rhythm stayed stubbornly in place. When he placed a student on a 12‑hour cycle, the subject’s temperature curve again refused to follow. Kleitman even tried less‑radical alterations, placing himself and several students on cycles lasting 21 or 28 hours. Some students were able to acclimatize. But Kleitman’s temperature stuck to the 24‑hour cycle, and he felt more irritable and exhausted with each altered day.
Why couldn’t he adjust? Were his cortical circuits less adaptable than those of other subjects, or were they more sensitive to external stimuli? It was impossible to tell without controlling for such stimuli, including the cues of sunset and sunrise. Hoping to eliminate those variables, Kleitman headed to Kentucky.
At their camp — a vast chamber about a quarter-mile from the cave entrance, 2,500 feet long by 150 feet wide — it was imperative the researchers receive no sensory cues to the cycles of the 24‑hour world. Richardson had also found it impossible to adjust to a 28‑hour schedule in previous experiments. The question was whether the pair would do better underground.
Kleitman and Richardson kept to a strict regimen, attempting to sleep from 12:00 a.m. to 9:00 a.m. Sunday; 4:00 a.m. to 1:00 p.m. Monday; 8:00 a.m. to 5:00 p.m. Tuesday, and so on — nine hours in bed and 19 hours out for each cycle. Cave workers brought meals, mail, newspapers, and smoking supplies twice a day, and spirited away the contents of the chamber pots. Portable generators powered the scientific instruments and gasoline lamps supplied the light, casting shadows on their specially built beds and bedside tables, stacked with equipment. To pass the time, they organized data, wrote letters, read periodicals and books, brushed up on bridge rules, or took brief walks around the cavern. With the air a constant 54 degrees Fahrenheit, they wore overcoats, hooded sweaters, galoshes, and two layers of woolen pants to ward off hypothermia. Every two hours, they measured their temperatures, relying on alarm clocks to rouse them during sleep periods. As they lay dreaming, the motion detectors kept vigil, disgorging ticker tape into a box beside each bunk.
For five 28‑hour cycles, events in the cave went precisely as planned. But despite their desire for secrecy (Kleitman had requested that the project be carried out discreetly), the newspapers got wind of the expedition, and a swarm of reporters descended. On June 10, Mammoth Cave manager W.W. Thompson sent a note with the food delivery, begging Kleitman to allow a visit from a representative of the press. Kleitman consented to an interview, and a correspondent of the Associated Press arrived the next day.
The 700‑word dispatch turned out to be a humdinger, full of fascinating science and colorful details. “Neither man has shaved in a week,” the correspondent reported. “There’s no sunrise or sunset to disturb them … yet they find themselves still unconsciously guided by the twenty‑four‑hour cycle that has ruled all their lives.” He explained the project’s rationale with admirable clarity and described the explorers’ living conditions down to the cigarette packs scattered across the breakfast table. The men assured him that their spouses had no reason to worry; the only discomfort was a slight dampness to the bedsheets. “When wives have crazy husbands, they get used to their doing things like this,” Kleitman said with a laugh, “and we feel that we may contribute something to the knowledge of men’s reactions.”
The story was picked up across the country, and more reporters came calling. When Kleitman received a request to shoot a newsreel on the last day of the expedition, the scientist again said yes. The clip, which was shown in thousands of movie theaters, opens with a title frame: STUDYING MYSTERY OF SLEEP, SCIENTISTS LIVE MONTH IN CAVE. “An alarm clock calls time on a unique experiment,” an announcer states as Richardson awakens and Kleitman extracts a length of tape from the bedside box. “Two Chicago university experts, who in the service of science have been living in the depths of Mother Earth for more than a month, end their test.” Then comes a close‑up of Kleitman. “We were entirely successful in our undertaking,” he says, “but want it known that it is in no way a stunt or an act of endurance or perseverance, but a bona fide scientific experiment.”
The newsreel is in some ways misleading. Contrary to the scientist’s declaration, the experiment was not entirely successful. Once again, Kleitman had been unable to acclimatize to a 28‑hour schedule while his student had done so within two weeks. Kleitman’s body temperature continued on its 24‑hour curve, and he always began feeling sleepy around 10:00 p.m. — even when that time fell during the morning or afternoon of the artificial day. He later suggested that age or other variations among individuals accounted for the difference between his and Richardson’s adaptability. The sample size, he admitted, was too small to draw conclusions.
Yet such details did nothing to diminish the story’s appeal. On July 7, when the men climbed the 71 steps from the cave entrance to the surface, dozens of journalists, park officials, townspeople, and tourists thronged to greet them.
What struck Kleitman most sharply, he told the newspapers, was the fragrance of the forest — a pleasant shock after more than a month with little to smell but dank stone, tobacco smoke, and bodily excretions. The clamor of the crowd must have been startling, too, as well as the commotion of bats among the oaks and hickories and the brilliance of the sky. The headlines were gleeful: SCIENTIFIC CAVEMEN EMERGE … SCIENTISTS QUIT PLAYING MOLES IN SLEEP STUDY … OLD WORLD LOOKS STRANGE AFTER A MONTH IN CAVERN.
To many Americans, the cave experiment was little more than a novelty. Its repercussions, however, can be felt to this day. Kleitman’s eccentric expedition would inspire a spate of experiments that revolutionized our understanding of how sleep‑wake rhythms function. More fundamentally, his emergence from Mammoth Cave marked a new era: The science of sleep had come out from underground.
With one stroke, Kleitman convinced millions of people that sleep was of “major interest” — and that it was a legitimate scientific field. While newspapers described how his “ten‑
thousand nights of scientific experiment” had “upset many popular ideas about the technique of sleeping,” Kleitman cemented his preeminence in 1939 with the publication of the first true textbook on sleep science — a 638‑page opus that would serve as the field’s bible for decades. Kleitman spent six years writing the tome, which covered virtually every study since the 1910s, including his own.
Building on this foundation, Kleitman attributed nearly every aspect of the sleep‑wake cycle to choices, behaviors, and sensory inputs rather than “some general properties of protoplasm” (that is, biological rhythms). What makes people feel the need to sleep after an extended period of wakefulness? Fatigue of the cerebral cortex — and of the muscles of the eyes, face, and neck, Kleitman said. What drives diurnal temperature rhythms? Different levels of mental and muscular activity during sleep and waking — though the temperature curve may persist through force of habit even when sleeping patterns change.
Kleitman’s text decisively established him as his field’s foremost authority. In its pages, researchers could find a set of facts, concepts, problems, and techniques to serve as points of departure for new explorations. The discipline was developing a sense of intellectual coherence, as well as a modicum of influence. Yet its grasp of sleep’s underlying mechanisms — and its ability to actually improve people’s lives — remained limited.
Not until years later were Kleitman’s theories proven wrong — in part due to discoveries concerning the “general properties of protoplasm,” as well as new findings in neuroanatomy and electrophysiology. Today, scientists see sleep as part of a circadian cycle, a built-in biological clock tied to the 24‑hour day. But without Kleitman’s work, blazing a trail for the thousands of sleep scientists who followed in his footsteps, this knowledge might’ve remained in the shadows — trapped in a world as deep and dark as the caves of Kentucky.
From the book MAPPING THE DARKNESS: The Visionary Scientists Who Unlocked the Mysteries of Sleep by Kenneth Miller. Copyright © 2023 by Kenneth Miller. Reprinted by permission of Hachette Books, an imprint of Perseus Books LLC., a subsidiary of Hachette Book Group Inc. New York, NY, USA. All rights reserved.
