Nathaniel Kleitman was a faculty member of the Department of Physiology and made significant contributions to modern sleep research. In 1938, Kleitman conducted a sleep research study with graduate student, Bruce H. Richardson, in Mammoth Cave of Kentucky over the course of 32 days. During the experiment, Richardson and Kleitman adapted to a 28-hour day under “uniform conditions of temperature, illumination, and quiet of the cave.” Their experiment demonstrated that the human bodies roughly maintains a 24 hour temperature cycle even with the absence of external cues. Kleitman wrote Sleep and Wakefulness in 1939, an account of the Mammoth Cave study findings.
Here is some news footage from that time about the experiment.