On 16 July 1962, a 23-year-old French geologist named Michel Siffre climbed down into the Scarasson cavern, a glacial cave in the Ligurian Alps on the French–Italian border, and stayed there for about two months. He had no clock, no calendar, and no daylight. He ate when he was hungry and slept when he was tired. A one-way telephone line ran to a small team at the entrance: he could call up to them, but they could not call down to him, and they had been told never to mention the time or the date.

When the surface team called down, in mid-September, to say the experiment was over, Siffre did not believe them. By his own count it was around 20 August. He thought he had nearly a month still to go.

That gap is the whole story, and it is worth being precise about what it does and does not show. This was one man, experimenting on himself, in conditions he largely controlled. Siffre was a geologist and caver by training, not a physiologist; the field now called human chronobiology barely existed in 1962. His stay is remembered less because it settled a question than because it made one vivid enough to study properly. Read it as the opening of an inquiry, not as a result that stands on its own.

What he actually did

Siffre had first gone to Scarasson to study its underground glacier and planned a stay of about two weeks. He extended it, a decision he later described in a 2008 Cabinet interview as the idea of his life. He said the space race partly inspired him: long isolation from ordinary day and night made human timekeeping worth studying.

The cavern sat roughly 130 metres below the surface. It was cold, close to freezing, and damp. He set up a tent, and each time he woke, ate, or prepared to sleep he telephoned the surface so the team could log the moment against a clock he was not allowed to see. Over roughly two months — accounts differ by a day or two on the exact length — that log became a record of a human being keeping time with no external reference at all.

Two different things drifted, and they are worth keeping apart

The popular version of this story collapses into a single claim: Siffre “lost track of time.” Two distinct things happened, and conflating them is where most retellings go wrong.

The first was his body’s daily rhythm. Left to itself, it did not descend into chaos. It kept running a cycle close to a normal day — by most accounts a little over 24 hours, often cited as around 24 hours and 30 minutes. Because that cycle ran slightly long, it slipped a little further behind the real world each day, the way a watch losing a few minutes an hour drifts off the hour over weeks. His body went on making days; they just were not quite Earth’s days.

The second was his conscious sense of how much time had passed, and this drifted far more dramatically. In a test he ran on himself, he would count slowly to 120, trying to mark off two minutes. By the later part of his stay it was taking him closer to five. His estimate of a short interval had stretched to less than half of real time — the same distortion, scaled up across two months, that left him roughly a month short in his running count of the days. When he emerged he described himself, in a phrase he used often afterwards, as feeling like “a half-crazed, disjointed marionette.”

So the body kept reasonable time while the person carrying it lost the thread. That split — between a rhythm that persists in the dark and a conscious clock that does not — is the durable thing the 1962 stay pointed to.

What it did not prove

Accounts of the experiment differ on the exact size of the counting error, with some putting it near a month and others closer to two weeks. The direction is not in dispute; the precise figure rests largely on Siffre’s own reporting, including his 1963 book about the stay and interviews he gave for decades afterwards. Much of the vivid first-person detail comes from him, recalled later, rather than from an independent observer beside him in the dark. That does not make it wrong. It does mean the drama and the data come from the same source, and it is worth knowing that.

The idea that the human internal clock naturally runs near 25 hours emerged from early isolation studies, including work by Siffre, Jürgen Aschoff and Rütger Wever. A 1999 Science paper led by Charles Czeisler used controlled lighting and estimated the intrinsic human circadian period at about 24.18 hours.

The likely reason for the discrepancy is mundane and important: subjects in the early cave and bunker studies controlled their own artificial lighting, switching lamps on and off as they pleased. Self-selected light exposure is now known to push the clock around, and it appears to have stretched the older estimates. This is the ordinary way science works — a striking early observation, later pinned down under stricter conditions — and it is a better description of Siffre’s legacy than any single figure.

The 48-hour cycle belongs to a later chapter

One detail attaches itself to the 1962 story that does not belong there: the claim that Siffre’s body settled into a 48-hour cycle, roughly 36 hours awake followed by a long sleep. He did not do that in Scarasson. That pattern turned up later — in some of the volunteers he sent underground through the 1960s, and in his own second major stay, in 1972, when he spent more than six months in Midnight Cave near Del Rio, Texas, with support from NASA. Even then, by his own account, he caught the 48-hour rhythm only in two stretches, not steadily. The long-cycle finding is real and strange, but it is a finding from those longer experiments, not from the two months this article is about.

Why it still matters

Siffre’s method — one committed amateur, sealed in the dark, keeping his own notes — is not how this question is studied now, and for good reason. His 1972 stay in Texas left him close to breaking off partway through, and roughly $100,000 in debt, and it kept him away from the field for years. When he returned to a cave in southern France to see in the new millennium, near the end of 1999, he lost the count again, uncorking his champagne several days late.

The experiment helped bring human chronobiology into public view. Decades later, the 2017 Nobel Prize in Physiology or Medicine recognised discoveries of the molecular mechanisms controlling circadian rhythms.

Michel Siffre died in Nice on 25 August 2024, at 85. His first two months underground had already settled the part that counts. In the dark, with nothing to mark the days, the body keeps making time — and the person carrying it has no idea what time it is.