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.
