Caveman Time

from an interview with Michel Siffre

"…Also, there were two tests I performed every time I called the surface. First, I took my pulse. Secondly, there was a psychological test. I had to count from 1 to 120, at the rate of one digit per second. With that test we made a great discovery: it took me five minutes to count to 120. In other words, I psychologically experienced five real minutes as though they were two.

The psychologist Elizabeth Loftus conducted an experiment in which she showed people a filmed scene of a bank robbery and asked them to estimate its duration. They overestimated by 500 percent. It seems that our subjective experience of time is highly variable. In the absence of clocks, how did you feel time’s passage?

There was a very large perturbation in my sense of time. I descended into the cave on July 16 and was planning to finish the experiment on September 14. When my surface team notified me that the day had finally arrived, I thought that it was only August 20. I believed I still had another month to spend in the cave. My psychological time had compressed by a factor of two.
…

When you were underground, entirely isolated from any artificial measurements of time, your body slept exactly when it wanted to, and for exactly as long as it wanted to. It might be said that you achieved perfect sleep. What was that like?

My sleep was perfect! My body chose by itself when to sleep and when to eat. That’s very important. We showed that my sleep/wake cycle was not twenty-four hours, like people have on the surface on the earth, but slightly longer—about twenty-four hours and thirty minutes. But the important thing is that we proved that there was an internal clock independent of the natural terrestrial day/night cycle. Interestingly, during the subsequent experiments I did with other research subjects, all of the people in the caves showed cycles longer than twenty-four hours. In fact, it became common for them to achieve cycles lasting forty-eight hours: They would have thirty-six hours of continuous activity followed by twelve to fourteen hours of sleep. After we made that discovery, the French army gave me lots of funding. They wanted me to analyze how it would be possible for a soldier to double his wakeful activity.
…

Did you ever succeed in catching a forty-eight-hour cycle?

Yes. In the 1972 experience in Texas, there were two periods where I caught the forty-eight-hour cycle but not regularly. I would have thirty-six hours of continuous wakefulness, followed by twelve hours of sleep. I couldn’t tell the difference between these long days and the days that lasted just twenty-four hours. I studied the diary I kept in the cave, looking cycle by cycle, but there was no evidence that I perceived those days any differently. Sometimes I would sleep two hours or eighteen hours, and I couldn’t tell the difference. That is an experience I think we all can appreciate. It’s the problem of psychological time. It’s the problem of humans. What is time? We don’t know."

full article here