In 1989 a young Italian woman named Stefania Follini spent 130 days - more than four months - in a sealed cave in Carlsbad, New Mexico. She had volunteered for an experiment focused on learning how the human body might respond to long periods of isolation during interplanetary travel. While underground, she slept ten hours at a time but often stayed awake for up to twenty-four, and her menstrual cycle completely stopped. She had so lost track of days that when Maurizio Montalbini, the Italian sociologist who directed the experiment, contacted her via computer to alert her that her four months were up, she was shocked. She thought she had been underground for only half that time.
A few years later, Montalbini himself performed an even more dramatic experiment showing how our disengagement from solar rhythms influences our sleep cycle and our perception of passing time. He spent an entire year in a cave. In the world above, 366 days and nights came and went while Montalbini stayed underground. One of his primary goals was to record his subjective response to the loss of solar cues, which meant that, like Follini, he took no clocks or calendars with him. When fellow scientists came for him after his year completely removed from the natural cycle of sunlight and darkness, he thought he had spent only 219 days underground.
But in October 2006 he entered another cave, this time planning to spend three years away from the regulating light of the sun.
(This experiment was cut short. See his wikipedia entry...)