When Josie Laures came out of her cave on March 12, 1965, she thought it was February 25. A few days before Antoine Senni came out of his cave, on April 5 that same year, he thought it was February 4.

The two cave explorers emerged from their holes in the French Alps, near Nice, 50 years ago.  Each of them set the then-world record for time spent alone in a cave—Laures set the female record at 88 days, and Senni the male record at 126 days—as part of an experiment to see what the effects of extreme isolation and loneliness would be on their bodies and minds.

While in their caves, which were separate but just a few hundred yards apart from each other, Laures and Senni kept in touch with researchers at a control point, who tracked their sleeping and eating habits, as well as memory and vital signs. The researchers did not, however, give Laures or Senni any clues about how time was passing up on the surface. By the time they emerged, wearing dark goggles to spare their cave-accustomed eyes from the sunlight reflecting on the alpine snow, they had lost weeks of time, by their own reckoning.

In chronobiology, the study of biological rhythms and how they relate to the environment, the German word zeitgeber means “synchronizer.” Natural light is the best-known, though not the only, zeitgeber that syncs human sleep patterns up with the Earth’s 24-hour day. Absent any cues from sunlight or even from clocks, Laures’s and Senni’s sleep schedules got wacky—sometimes without them realizing it. A Chicago Tribune article titled “Sleepy Caveman Calls it Quits” proclaims Senni “a great sleeper, sometimes nodding off for 30 hours at a time and waking up believing he had simply had a short nap.” (In years since, researchers have found that people often slip into 48-hour sleep cycles when isolated from the environment.)

This being the era of the space race, and just a few years after President Kennedy’s call to put a man on the moon, the subjects, researchers, and NASA alike saw the cave jaunt as a window in to what astronauts might experience on long, lonely space missions.

The experience was definitely trying. Upon her exit, Laures told the Associated Press:

I am so happy to have lasted it out, that I have forgotten everything. I can tell you though that it became very difficult toward the end and I felt terribly worn out … At the start of my stay I read, and then I lost the desire. I didn't suffer from the cold. I was well heated in my little tent. My tape recorder refused to work the first few days, but later I managed to repair it and I listened to music. Outside of that I knitted, and knitted some more, and looked forward to the time when I would finally see the sun.

But aside from that, the many old AP and Reuters articles I read about Senni and Laures didn’t say much about their physical and mental states post-emergence. Senni was apparently pronounced “in very good physical shape” by someone at the control point. And Michel Siffre, the speleologist who oversaw the experiment, told the AP that Laures was “in perfect health” after her stay, according to the hospital tests, though he noted that she had some temporary color-blindness, and trouble getting back to a normal sleep pattern, which does not sound perfect to me.

Later studies on isolation have found not only effects on sleep cycles but anxiety, hallucinations, and a decline in mental performance. These were sensory deprivation studies, though, and at least in the caves, the subjects were welcome to read or knit or listen to music.

Siffre’s isolation studies, of which Laures’s and Senni’s was but one, drew criticism as well as admiration.

“Some people think he is a bad boy,” the chronobiologist Franz Halberg told The Los Angeles Times in 1988. “But Siffre does what nobody else will do. He has, by far, the longest records of people in isolation. Others who have studied similar situations have done it for weeks; he has done it for months.”

Siffre also experimented on himself. In 1962, a couple years before Laures and Senni descended into their own stalagmite-studded holes, Siffre spent two months in a glacial cave in the Alps. Ten years later, in 1972, he did an even longer stint—six months in a cave near Del Rio, Texas.

“Physically it was not tiring, but mentally it was hell,” Siffre told the German magazine Der Spiegel, in 2014.

Siffre was so lonely in the Texas cave that he attempted to catch a mouse to keep as a pet. According to Der Spiegel, he spread jam on the floor of the cave, and while the mouse was licking it, brought a dish down over it to trap it. But he aimed poorly, and when he lifted the dish, the mouse had been crushed to death by the dish’s edge. “Desolation overwhelms me,” he reportedly wrote in his diary.

Laures, too, turned to rodents for friendship during her time in the cave, but with happier results. “A white mouse was her sole companion during the three-month ordeal,” the AP reported. “It came through the experiment in good shape.”