MAMMOTH CAVE — Seventy-five years ago, two men researching natural rhythms of the human body spent about a month inside Mammoth Cave.

From June 4 to July 6, 1938, they wanted to see if a body could be configured to an artificial 28-hour day with nine hours of sleep between days.

This past week, modern researchers of chronobiology explored the exact spot in the cave. “These are the scientific descendants and the early discoveries of what we call chronobiology,” said Doug McMahon, associate director of education and training at the Vanderbilt Brain Institute in Nashville. He also serves as director of graduate studies in neuroscience at Vanderbilt University.

Chronobiology, which concerns natural physiological rhythms, is the scientific discipline that was spawned in part by the research of Nathaniel Kleitman and Bruce Richardson from the University of Chicago. It was there in 1938 that the men ate fried chicken prepared by cooks at the Mammoth Cave Hotel and read the newspaper by lantern, living in a constant 54-degree environment. The chill required some clothing layers, according to an old newsreel of the event. The students and researchers who visited the cave are attending the Chronology Summer School at Vanderbilt. 

Till Roenneberg, professor of chronobiology at the Institute of Medical Psychology at Ludwig-Maximilians University in Munich, Germany, said it’s the first time a chronobiology summer school, usually in Europe, has been in America since the 1960s. 

Roenneberg said it is amazing, given the nascent state of chronobiology knowledge in 1938, the insightfulness of the researchers who were looking into functions of the human body. Ironically, while Richardson, a man in his 20s, was able to adjust to the new sleep pattern, the older Kleitman had difficulty. Kleitman, because only two subjects were used, didn’t publish his findings immediately, but later wrote a book about early sleep research. When the men wanted to sleep, they simply extinguished the lanterns that lit their rock-walled home. The hotel, besides providing food, also provided the furniture they used. The beds were configured into bowls to keep rats out during complete darkness.

Before venturing into Mammoth Cave’s Rafinesque Hall, not far from the historic entrance to the cave, about 140 feet underground and a 15-minute stroll, the summer school contingent heard from Dr. William Schwartz of the Department of Neurology at the University of Massachusetts Medical School, who discussed Kleitman and Richardson’s project and how it relates to modern-day science. Kleitman wrote “Sleep and Wakefulness” in 1939 and is considered the “father of modern sleep research,” Schwartz said.

Besides the cave experience, Kleitman also lived north of the Arctic Circle and spent two weeks in a submarine as he and Richardson explored wakefulness.

The human brain contains a mechanism that actually sets our body clock. Through stimuli by light entering our eyes, a complex series of biochemical reactions occur to affect body temperature and activate the body’s internal clock – the circadian clock. Schwartz said he recently worked with a 74-year-old man in Massachusetts who claimed to have “an upside down circadian clock.” The sleep disorder had continued for two years and was accompanied by anxiety issues.

The man was advised to try light therapy, using a flashing light at night intended to activate and re-set his body clock and a hormone timed to work into his system. The man never tried any of the therapies, but told Schwartz that he felt better simply because the doctor had told him he wasn’t crazy. “I can live with that,” the patient told the doctor.

The clock genes function like a thermostat, measuring temperature and stimuli and adjusting the body accordingly. Chronobiology research has shown that as certain hours of the day, the human body is better prepared for activities. For example, Schwartz said, it is better to go to the dentist in the early afternoon, because the body’s pain tolerance is greater at that time.

The meals from the hotel that the men ate “were their greatest single pleasure” of the experience, noted Colleen Olson, a Mammoth Cave park guide, who has also researched the 1938 sleep experiment. She said in addition to the fried chicken, the men also ate “hickory-smoked country ham.” The cave had not yet come under the purview of the U.S. Park Service, but arrangements were made to conduct the study at Mammoth Cave through Professor J.H. Bretz of the Department of Geology at the University of Chicago.

“This is a real treat for us,” McMahon said of the cave visit. 

Schwartz said the body clock recognizes the time of day like a sundial and measures lapse of time like an hourglass. When sections of a rat’s brain that regulated the animal’s body clock are put under a microscope as a chemical is used to track the electrical impulses that result from the biological reactions, the brain glows in a rhythm.

“We have the same kind of internal clock that the mouse does,” Schwartz said.

Chronobiology is a growing scientific discipline, Schwartz said, and can be applied to studies of obesity and how what people eat can change their body chemistry.

“I love this cave,” said Paula Cormandy, who works for Eastern National and manages Mammoth Cave’s bookstore. “I didn’t know about the sleep study.”

Dr. Rick Toomey, director of Mammoth Cave’s Learning Center, told the visitors that the cave hosts 120 species of wildlife – 40 of those have physiologically adapted to the cave through changes in their bodies and the length of their life cycles. Study continues on the cave’s wildlife contingent.

“This is a perfect illustration of the value of our national parks,” McMahon said of the visit to the site where a modern-day science was born 75 years ago deep below the Kentucky hills.

— To access the newsreel, go to www.weirdexperiments.com/19201939.htm and scroll to the bottom of the page.

— Chuck Mason covers education for the Daily News. Follow him on Twitter at twitter.com/bgdnschools or at bgdailynews.com.

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