On the morning of Sept. 14, 1962, 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.
His death was announced by his family in a statement. In a separate statement, the Society of French Explorers said the cause was pneumonia. He was 85.
Mr. Siffre’s work drew inspiration — and, eventually, government funding — from the Cold War and the space race. His 1962 adventure came immediately before the Cuban missile crisis, which spurred Americans to dig fallout shelters in their backyards and wonder what a new life below ground would be like.
The French Army wanted to know if it could keep soldiers awake longer. NASA wanted to know how the sleep cycles of astronauts would be affected on a mission to Mars, or beyond. Both of them supported his later research.
Through the 1960s, Mr. Siffre oversaw more trips underground by volunteers, all of them similarly cut off — he chose caves without bats, for example, because their nocturnal habits were a clue to the time of day.
His experiments ran longer and longer, and the results were astounding. Without the sun’s influence, his subjects experienced wildly different circadian rhythms; some slept for 12 hours, then had no problem staying awake for 36 — though they experienced those 48 hours as 24.
He returned to the subterranean in 1972, staying for six months in a cave in South Texas. Electrodes attached to his chest and head measured various vital statistics, and he spent his time running a long protocol of tests on himself.
At one point the solitude got to him; he ripped the sensors off and almost called off the mission. Ten days later, he calmed down, reattached them and finished his stay — which at the time was a world record for time spent underground. Once again, he thought he’d been down for only a short time.
“When we told him, ‘Hey, OK, it’s time to come out,’ he didn’t believe us,” Greg Passmore, a caver who assisted on the project as a teenager, said in an interview. “He said, ‘You know, you’re just messing with me.’”
Mr. Siffre emerged depressed and in debt. His wife, Nathalie, had decided to leave him. Even with government funding, his project had cost him a small fortune.
At the time, he also faced criticism from cavers, some of whom felt he had disturbed fragile subterranean ecosystems, and skepticism from biologists, who doubted his work as a nonspecialist.
Over time, though, Mr. Siffre’s research has proved invaluable to other researchers, and it stands as a pivotal contribution to chronobiology — a field in which three scientists, Jeffrey C. Hall, Michael Rosbash and Michael W. Young, won the Nobel Prize in Biology in 2017, for discovering the genetic basis for our internal clocks.
Michel Augustin Francis Siffre was born on Jan. 3, 1939, in Nice. His father, Jean, was a winemaker before World War II, a prisoner of war during the conflict and a tax official afterward. His mother, Lucie (Roques) Siffre, managed the home.
He received an undergraduate degree in geology from the University of Paris in 1960.
Mr. Siffre is survived by his brother, Alain.
After his 1972 descent, Mr. Siffre turned to writing books and lecturing about caves. He spent some time in Sri Lanka, looking for caverns bearing precious stone, and in Guatemala, exploring subterranean sites for evidence of pre-Columbian settlement.
He returned below ground once more, after hearing that John Glenn, the first American to orbit the earth, had returned to space in 1998, when he was 77.
He entered a cave in November 1999 and emerged in February 2000. He had taken foie gras and champagne along to celebrate the new millennium, though once again his internal clock failed him — what he thought was New Year’s Day was in fact Jan. 4, 2000.
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