He Vanished Into a Cave for 63 Days—And Emerged With a Scientific Breakthrough No One Saw Coming

In the summer of 1962, a young French geologist named Michel Siffre descended into a glacial cave in the French Alps with no clock, no calendar, and no contact with the outside world. When he emerged 63 days later, he didn’t know the date, couldn’t estimate how much time had passed, and described himself as feeling like a “half-crazed, disjointed marionette.”

What had started as a geological expedition became a pioneering experiment in human biology, laying the foundation for the scientific field of chronobiology—the study of the body’s internal clock. Siffre had originally planned to study a newly discovered glacier in Scarasson, a remote and icy cave system located 130 meters below the surface.

His initial goal was to spend just fifteen days underground. But after further reflection, he decided that two weeks would be insufficient for a meaningful investigation. He expanded the expedition to a full two months, designing what would become one of the most extreme self-experiments in scientific history.

“I decided to live like an animal,” Siffre told New Scientist in 2018. “Without a watch, in the dark, without knowing the time.”

Life in Total Isolation

The conditions were brutal. Temperatures inside the cave hovered below freezing, and humidity levels reached 98 percent. Siffre had minimal equipment and was forced to endure cold, damp clothing and constant discomfort.

“My feet were always wet, and my body temperature got as low as 34°C (93°F),” he said.

He had no way to track time, no artificial lighting schedules, and no sensory cues to help him maintain a regular rhythm. His only connection to the surface was a radio line, but it transmitted only basic updates—never time or date information.

Despite the harsh environment, Siffre began recording his mental state, sleep patterns, and overall well-being. He kept a detailed diary and performed psychological tasks, including counting exercises.

The data he collected would later offer a rare glimpse into the effects of extreme temporal isolation. One key finding was that his sense of time dramatically slowed down. Counting to 120, for instance, took him five minutes instead of two.

“My psychological time […] compressed by a factor of two,” he explained.

A Body Freed from Time

Perhaps the most significant discovery from the expedition was the realization that the human body operates on its own internal schedule, separate from the external cycle of day and night.

In the absence of sunlight, clocks, or schedules, Siffre’s circadian rhythm began to stretch. At first, his biological day lengthened from the standard 24 hours to about 24.5 hours. But in a follow-up experiment a decade later, his internal cycle extended dramatically.

“I would have thirty-six hours of continuous wakefulness, followed by twelve hours of sleep,” he noted.

What startled researchers even more was that Siffre himself was unaware of these changes.

“I couldn’t tell the difference between these long days and the days that lasted just twenty-four hours,” he said.

Reviewing his own journal entries, he found no evidence that he consciously noticed the shift in his routine. It became clear that humans, when deprived of external cues, do not maintain a strict 24-hour cycle. His findings were the first to demonstrate that our internal clocks could be uncoupled from environmental signals.

From Fringe Idea to Scientific Breakthrough

Siffre’s approach was initially met with skepticism. He was seen by many as a geologist dabbling in biology without formal training. Critics called his methods sensational and potentially hazardous, especially considering the fragile ecosystems he might disturb by spending extended time underground.

But over time, his bold and unconventional experiment would prove immensely valuable. The data he gathered laid the groundwork for an entirely new discipline—human chronobiology. This emerging field would go on to shape our understanding of sleep disorders, shift work, jet lag, and even the timing of cancer treatments.

The implications of his work were not lost on institutions with a vested interest in human performance. The Cold War space race and nuclear submarine programs were both in full swing. NASA and the French military quickly saw the potential in Siffre’s findings.

“It was the Cold War,” he recalled. “Not only was there a competition between the US and Russia to put men into space, but France had also just begun its nuclear submarine program.”

With so little known about how the human body functioned in isolation, the military and aerospace sectors turned to Siffre’s pioneering research.

“NASA analyzed my first experiment in 1962 and put up the money to do sophisticated mathematical analysis,” he said.

Unpredictable Rhythms in Extreme Environments

Siffre wasn’t the only one to experience drastic biological shifts in isolation. In the years following his initial expedition, other volunteers participated in similar cave-based studies, some of them working directly with him.

All of them reported irregular and unexpected patterns in sleep and wakefulness. One individual experienced alternating periods of 25-hour “days” and 12-hour “nights.” Another slept for 33 hours straight, prompting surface researchers to wonder if he was still alive.

“It was the first time we’d ever seen a man sleep for that long,” Siffre later said.

Despite the physical and psychological toll of these experiments, Siffre remained a passionate advocate for underground exploration and discovery. “Caves are a place of hope,” he reflected years later.

“We go into them to find minerals and treasures, and it’s one of the last places where it is still possible to have adventures and make new discoveries.”

His work may have begun with geology, but it ultimately reshaped biology, opening up new frontiers in our understanding of time, perception, and the human body.