How a Cave Expedition Led to the Accidental Discovery of the Human Biological Clock

In 1962, French geologist and speleologist Michel Siffre set out on what he believed would be a routine scientific study of a glacier buried deep in a cave. What unfolded during that journey, however, would change our understanding of human biology forever.

At just 23 years old, Siffre was fascinated by the geology and natural science of caves. He chose the Scarasson chasm, a glacial cave located 130 meters (about 426 feet) underground in the Maritime Alps along the French-Italian border, as the site of his expedition. Initially, his goal was simple: spend 15 days underground to study the cave’s physical characteristics. But his curiosity soon led him to propose a much bolder experiment—one that would isolate him completely from the outside world and all references to time.

A Life Without Time

Siffre decided to live in the cave with no access to clocks, watches, sunlight, or any indicators of time. He brought with him basic survival supplies—non-perishable food, a tent, and a battery-powered lamp—but no tools to measure the passing hours. He communicated with a team on the surface via a telephone line, only to report when he woke up, when he ate, and when he went to sleep. His team, however, never gave him any updates on the actual time or date.

What began as a 15-day stay turned into an astonishing 63-day isolation experiment. During that time, Siffre’s perception of time began to unravel. Because he had no natural light or external cues, his sleep-wake cycle began to shift. By the end of the experiment, he believed it was August 20 when it was actually September 14—almost a full month off.

The Accidental Discovery of Circadian Rhythms

Siffre’s experience became a turning point in science. His team realized that, even without external time references, Siffre’s body continued to follow a consistent rhythm. Although his internal “days” began to lengthen—sometimes stretching up to 48 hours—his body still maintained regular cycles of wakefulness, eating, and sleep.

This was the first major modern insight into what we now call the circadian rhythm—the internal clock that governs many biological processes in humans and other animals. The word "circadian" comes from the Latin circa diem, meaning “about a day.” This internal clock, located in the brain’s suprachiasmatic nucleus, controls everything from sleep patterns to body temperature, hormone production, and even cognitive performance.

Siffre’s unintentional dive into this field of study helped lay the foundation for chronobiology, the scientific study of biological rhythms. His findings were so significant that they attracted attention from NASA, the U.S. military, and various universities, all interested in understanding how humans adapt to environments where normal time cues are absent—such as outer space, submarines, or even long-haul flights.

Repeating the Experiment—and Going Even Further

Siffre didn’t stop with his first cave mission. In 1972, a decade later, he conducted another, even more ambitious experiment. This time, he descended into a cave in Texas and spent six full months underground, again with no access to time indicators. The results mirrored and deepened his original observations: humans have an innate timekeeping system that doesn’t strictly follow the 24-hour day we’re accustomed to.

These experiments also revealed something unexpected—prolonged isolation and the loss of time cues can have psychological effects. While Siffre maintained his sanity, he reported moments of depression, confusion, and anxiety. These emotional responses highlighted the importance of light exposure and routine in maintaining mental well-being.

A Lasting Legacy

Today, Michel Siffre is regarded as a pioneer of human chronobiology. His work has been crucial in the development of research on sleep disorders, jet lag, shift work fatigue, and even Seasonal Affective Disorder (SAD). Scientists now understand that disrupting the circadian rhythm—such as by working night shifts or frequently flying across time zones—can have serious consequences for both physical and mental health.

Siffre’s insights have also influenced the design of modern space missions, where astronauts must maintain regular schedules in environments without natural light. His research remains relevant in the development of treatments for insomnia, mood disorders, and metabolic health issues linked to disrupted sleep.

Michel Siffre never intended to become a chronobiologist. But by spending months in the darkness of the Earth’s depths, he inadvertently brought to light one of the most important discoveries about how humans function: that we all carry within us an invisible clock, ticking quietly beneath our consciousness, guiding every aspect of our daily lives.

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Do you think society underestimates the importance of circadian rhythms in areas like education, work schedules, or healthcare? What changes could we make if we took our internal clocks more seriously?