The very beginnings of circadian / sleep research

I recently came across this amazing video documenting of one of the first modern experiments on circadian timing and its control over our daily sleep/wake patterns. Dr. Nathaniel Kleitman and his graduate student, Bruce Richardson, settled for one month into the dark recesses of Mammoth Cave in Kentucky. They wanted to remove themselves from visual cues of the 24-hour hour world and see if they could adjust to a self-imposed 28-hour day; while their experiment was somewhat of a bust, Nathaniel Kleitman went on to become one of the foremost experts on sleep research.