On the morning of Sept. 14, 1962i1pera, reporters and onlookers began to gather around a hole in the ground, far up in the Maritime Alps between France and Italy. A few hours later, workers rigged a rope down into the darkness; soon they pulled out a small, sturdily built man named Michel Siffre.

He had been inside the cave, 375 feet down, for 63 days, with only a four-volt lamp for illumination. He wore dark goggles to limit the glare of the sun, and he had to be carried to a waiting helicopter.

This was no rescue: Mr. Siffre, a geologist, was conducting an experiment on himself, to see what would happen to his sense of time if he cut himself off from the normal day-night flow of life on the surface.

It turns out that a lot could happen: Time as he experienced had “telescoped,” he said. His circadian rhythm of wakefulness and sleep stretched from 24 to about 25 hours. And what felt to him like one month was in fact two on the surface.

“After one or two days, you don’t remember what you have done a day or two before,” he told Cabinet, an art and culture magazine, in 2008. “The only things that change are when you wake up and when you go to bed. Besides that, it’s entirely black. It’s like one long day.”

Mr. Siffre, who died on Aug. 25 in Nice, was a leading figure in the field of chronobiology, the study of how the human body understands time. Previous scientists had speculated that, contrary to the prevailing idea at the time, our internal clocks are independent of the solar cycle, even as we usually adjust to its influence. Through decades of experiments beginning with that 1962 descent, he proved it.

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