The Scientist Who Went Into a Cave to Discover Chronotype

On July 16, 1962, a 23-year-old French caver descended 130 meters underground, into a cave in the Alps, with no watch, no calendar, and no way of knowing whether it was day or night outside. When his team radioed that he had to come up because the experiment was over, he was convinced it was August 20. It was actually September 14. His body, left to its own devices for 63 days, had lost nearly a month along the way.

That man was Michel Siffre, and what he discovered in that cave (and in the two that followed, at ages 33 and 60) wasn’t just a laboratory curiosity. It was, in large part, the origin of a question that’s still open today: why does each person’s internal clock run at a slightly different speed, and what does that have to do with being a morning person or a night owl?

An experiment with a single subject: himself

Siffre wasn’t a psychologist or a doctor. He was a geologist, and he had gone down into the Scarasson abyss, in the Maritime Alps, to study an underground glacier. At some point during that expedition, a different idea occurred to him: what would happen if someone lived down there, completely isolated, for two months, with no time references at all?

Years later he summed up his own decision in a line that captures the spirit of the experiment well:

“I decided to live like an animal, without a watch, in the dark, without knowing the time.”

The conditions were simple to describe and brutal to sustain:

• No natural light, apart from a lamp he switched on and off himself.
• No clock or calendar of any kind.
• A single point of contact with the outside: a team on the surface he could phone whenever he decided to call, never the other way around, so they couldn’t give him any hint of the time up above.
• Food and supplies lowered on a pulley system, with no pattern he could use to orient himself in time.

What his body did when no one was telling him the time

The first thing that struck Siffre, and later the researchers who analyzed his data, was that his body didn’t fall apart. Quite the opposite: with no outside help at all, his sleep-wake cycles stayed remarkably regular.

“My sleep was perfect! My body chose by itself when to sleep and when to eat.”

That spontaneous cycle, carefully tracked by the surface team from his calls, wasn’t exactly 24 hours long. It ran closer to 24 hours and 30 minutes. That half-hour difference, trivial on any single day, kept accumulating day after day, and it’s the mathematical reason Siffre ended up living almost a month “extra” without noticing: each of his biological days was a little longer than a calendar day, so within a few weeks his internal clock and the wall clock had drifted completely apart.

This is the concept chronobiologists call the free-running period: the actual length of your sleep-wake cycle when you receive no external signal forcing you to align with exactly 24 hours. And here’s the detail that connects this anecdote directly to chronotype: that period isn’t identical from person to person. It runs close to 24 hours in most people, but some run slightly longer and some slightly shorter, and that difference, sustained day after day, is one of the most direct biological explanations for why some people naturally lean toward eveningness and others toward morningness.

The day that stretched to 48 hours

Ten years later, in 1972, Siffre repeated the experiment on a bigger scale: 205 days in Midnight Cave, in southern Texas, this time with more technical support and with NASA closely following his results, interested in how astronauts might cope on long missions with no clear time references.

What happened this second time around was even more striking. In some stretches of those 205 days, his cycle stopped hovering around 24 and a half hours and stretched much further, brushing up against 48 hours: nearly 36 hours awake in a row, followed by 12 to 14 hours of sleep. And the strangest part wasn’t that it happened, but how it felt from the inside:

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

In other words, for his body, isolated from any reference point, a 24-hour “day” and a 48-hour one felt equally normal. The subjective sense of what counts as “a day” doesn’t come from some infallible internal clock; it comes from the sleep-wake cycle itself, whatever its actual length happens to be.

Meanwhile, in a bunker in Bavaria

Siffre ran these experiments alone, almost as a personal feat. But the same idea, applied with far more rigor and at a much larger scale, was developing in parallel in Germany. Physiologist Jürgen Aschoff built a concrete bunker in Andechs, near Munich, and had 447 different volunteers, mostly students, live there between 1964 and 1989, in stretches of two to four weeks each, isolated from sunlight, clocks, radio, and any scheduled contact.

The results confirmed, with a sample size no single cave could offer, what Siffre’s experience had hinted at:

• Most people, with no external signal at all, settled into a cycle close to 24 to 25 hours.
• That cycle wasn’t identical from one volunteer to the next; there was real, measurable individual variation.
• In some participants, different body rhythms (like temperature and the sleep cycle) even ended up desynchronizing from each other, a phenomenon that led Aschoff to propose that the human biological clock isn’t a single clock at all, but several internal clocks coordinated with one another.

It’s probably the closest thing we have to a mechanical explanation for why chronotype varies from person to person: it’s not just a matter of habit or preference, it’s that each person’s internal clock, measured under conditions of complete isolation, literally runs at a slightly different speed.

Thirty-seven years later, another cave and a different result

The last piece of this story is, perhaps, the most human one. In November 1999, at 60 years old, Siffre shut himself underground again, this time in the Clamouse cave in southern France. This time the goal was explicit: to check how time perception changes with age, comparing his own results to those from nearly four decades earlier.

He was underground for the turn of the millennium. When he finally emerged, convinced the year 2000 had just begun, he discovered he was actually more than three days behind the real calendar.

You don’t have to stretch the reading too far to see the parallel with something we already know from other research: chronotype isn’t fixed for life. It changes with age, usually in a predictable direction. Siffre didn’t measure that shift in a population survey; he lived it in his own body, twice, 37 years apart, cave to cave.

What this means for your own clock

You don’t need to shut yourself in a cave, of course, to get a hint about your own free-running period. Almost everyone runs a much gentler version of this same experiment every weekend, without realizing it:

• On weekdays, the alarm clock and your obligations force your sleep schedule into exactly 24 hours, whether you like it or not.
• On free days, without that alarm, your actual sleep and wake times drift much closer to what your body would ask for on its own.
• How much your schedule shifts between one kind of day and the other gives an imperfect but real hint about which way your internal clock pulls when nothing is interrupting it.

Michel Siffre spent a good part of his adult life proving, cave after cave, something that can be summed up in a single sentence today: the time your body keeps and the time on the calendar are two different things, aligned by habit, not by nature. And the gap between them, small or large, is one of the pieces that best explains why your chronotype is what it is.

Sources

• Foer, J. (2008). Caveman: An Interview with Michel Siffre. Cabinet Magazine, 30.
• Wikipedia contributors. Michel Siffre. Wikipedia, the Free Encyclopedia.
• Wikipedia contributors. Bunker experiment (Jürgen Aschoff, Andechs). Wikipedia.
• Aschoff, J., & Wever, R. (1976). Human circadian rhythms: a multioscillatory system. Federation Proceedings, 35(12), 2326-2332.