For approximately one million years human beings have lived in a physiological rhythm determined by day and nightthat is, a rhythm of about 24 hours. So ingrained is this habit that a daily temperature cycle occurs, body heat being lowest (among people who normally sleep at night) in the early morning, highest in the early afternoon. Some time ago Physiologist Nathaniel Kleitman of the University of Chicago determined to find out whether the human mechanism could break away from this ages-old habit, adapt itself to a cycle of different length.
On June 4, Dr. Kleitman and Graduate Student Bruce Richardson entered Kentucky's Mammoth Cave, took up residence in a snug cavern 119 feet underground where for them day and night on the surface had no meaning. There they lived a 28-hour cycle, sleeping nine hours each 28-hour "day." There were only six of their long days in a calendar week. They had a regular routine of eating, sleeping, reading, writing, walking.
After 32 calendar days underground, the two scientists emerged last week. Results: Richardson adapted himself well to the long day, sleeping soundly at the prescribed periods and stretching his temperature cycle to one of 28 hours. Kleitman had much difficulty, his periods of wakefulness and sleepiness and his temperature cycle clinging to the 24-hour schedule. This indicated that ability to break away from the 24-hour rhythm, while not impossible, varies with different individuals. Perhaps age is a factor, since Kleitman is 43 and Richardson only 25.