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Dreaming Underground: The Mammoth Cave Sleep Study

In 1938, researchers from the University of Chicago ventured into the depths of Mammoth Cave to conduct what would ultimately prove to be a groundbreaking study for the field of sleep science. Two pioneering sleep scientists, Dr. Nathaniel Kleitman and his graduate student Bruce Richardson, embarked on a month-long experiment to study the fluctuations of wakefulness and body temperature. Their goal was to understand how the absence of a natural light-dark cycle influenced mental performance and biological rhythms.

A man uses a piece of equipment to take data readings while another man sleeps in a metal framed bed. — Researchers study sleep in the depths of Mammoth Cave.
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The Experiment

Two men wearing coats and holding lanterns stand near a large iron gate leading into a cave. — Dr. Kleitman and Richardson emerge from Mammoth Cave after spending a month underground.
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Goal of the Project

Dr. Kleitman aimed to determine if it’s possible to alter one’s circadian rhythm in the absence of natural environmental cues, such as daylight. To investigate his theory, he and his graduate student would venture into the depths of Mammoth Cave, where they would attempt to adjust their internal clocks to a 28-hour day instead of the typical 24-hour cycle.

Three men stand in a cave passageway near assorted metal-framed furniture. A man wearing a waitstaff uniform holds a metal food container and gives an envelope to one of the other men. — A waiter from the Mammoth Cave Hotel delivers lunch and mail to the scientists.
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A Unique Environment

Mammoth Cave, renowned for its vast subterranean passages with stable conditions, served as the perfect backdrop for this innovative experiment. The researchers resided in Rafinesque Hall, a large cave passageway near the Historic Entrance, where they meticulously recorded their daily routines, with a specific emphasis on how their bodies reacted in this unusual environment. Freed from the regulating influence of sunlight, they aimed to dissect the intricacies of the human circadian rhythm.

Two men stand near a wash stand in an underground cave passage. One washes his hands while the other man pours water from a large pitcher. — Kleitman and Richardson complete their daily routine underground.
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Conducting a long-term experiment in a cave environment, however, presented a unique set of challenges. The duo depended on help from those above for necessities like food, drinking water, and communication with the outside world. Staff from the Mammoth Cave Hotel played a key support role, delivering meals and mail to the researchers’ underground laboratory-turned-home. The cave’s constant humidity and cool average temperatures of around 54° degrees Fahrenheit (12° Celsius) also meant the researchers had to wear multiple layers of clothing with heavy coats to stay warm. About their temporary residence Richardson remarked, “I guess we missed hot baths more than anything else.”

Notable Findings

Body Temperature and Efficiency

One of the ways Dr. Kleitman and Richardson tracked circadian rhythm was by noting changes in body temperature. The findings yielded from the study were remarkable. The researchers discovered that body temperature fluctuates throughout the day, significantly impacting cognitive and physical performance. Notably, they found that a person’s peak efficiency often coincided with their peak body temperature. This information laid the groundwork for understanding the physiological basis of human productivity and performance.

Two men study books and papers while seated at a table with a lantern placed in the middle. — The scientists study by lanternlight in their underground laboratory.
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Individual Variability in Adaptation

A key observation from the study was the differing responses of the two researchers to the experiment. Richardson, who was 25 years old, was able to adapt smoothly to a 28-hour sleep-wake cycle. This flexibility in adjusting to a longer cycle showcased the potential for younger individuals to acclimate to drastic changes in their environments. In contrast, 43-year-old Kleitman remained anchored to a traditional 24-hour cycle, demonstrating the inherent variability in human adaptability.

Two men in an underground cave passageway, one stands while the other reads by lanternlight while seated at a table. — Dr. Kleitman and Richardson in Rafinesque Hall inside Mammoth Cave.
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Dr. Kleitman poignantly remarked, "It merely shows what has been known for a long time, that some individuals can change their habits with ease, while others find it very difficult to do so." This statement highlights the deeply personal nature of sleep patterns and the complexity of human biology.

Legacy of the Mammoth Cave Sleep Study

Portrait of a bearded man wearing a suit and tie. — Dr. Nathanial Kleitman sports a new beard following the month-long underground experiment.
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A Pioneer of Sleep Research

Following this trailblazing underground research experiment, Dr. Nathaniel Kleitman continued to contribute to the field of sleep science for decades. His work would eventually lead him to the University of Chicago and earn him the title “Father of Sleep Research,” thanks to his numerous groundbreaking discoveries, including the identification of Rapid Eye Movement (REM) sleep. The findings from the Mammoth Cave study not only reinforced the importance of sleep but also opened new avenues for understanding human physiology and psychology.

A group of 9 people wearing headlamps stand or kneel around papers spread on the floor in an underground passageway. — Researchers from the University of Chicago visit the site of the experiment in 2024.
Photo Courtesy of University of Chicago

Reawakening Research

In 2024, researchers from the University of Chicago once again descended into Mammoth Cave, this time for a short visit to commemorate the pioneering work of Nathaniel Kleitman and Bruce Richardson. The visiting team, armed with modern technology and fresh perspectives, sought to better understand how the cave's unique environment might shed light on circadian rhythms and sleep health. As they retraced the steps of Dr. Kleitman and Richardson, the researchers celebrated the profound impact of the original experiment on contemporary understandings of sleep, while envisioning new avenues for exploration in the ever-evolving field of sleep science.

Putting it All to Bed

The Mammoth Cave sleep study remains a fundamental milestone in the field of sleep research. By stripping away the conventional markers of time, Dr. Kleitman and Richardson highlighted the intricate ways our bodies respond to environmental cues, paving the way for future investigations into sleep patterns, performance, and individual differences. Their legacy endures in the ongoing exploration of sleep's vital role in human health and well-being.

Having served as a unique laboratory for this experiment that changed the course of sleep science and a variety of other experiements in the decades since, Mammoth Cave continues to reveal its secrets through modern-day research efforts. Its depths have sparked human curiosity for millenia and will continue to inspire a deeper understanding of our natural world for generations to come.