In 2021, fifteen volunteers sealed themselves inside a French cave without clocks, phones, or sunlight for 40 days, but when they emerged, most believed they had spent far less time underground

Imagine giving up your phone, your watch, and even sunlight for 40 straight days. That's what 15 volunteers did in 2021, walking into Lombrives Cave in the French Pyrenees for a project called Deep Time. According to the Associated Press report from PBS NewsHour, the group emerged on April 24, 2021, and project director Christian Clot said that in most participants' minds, they had entered the cave only about 30 days earlier, not 40. Ten days had effectively vanished from their sense of time.

Who was inside, and what life looked like
According to the same PBS NewsHour report, the group had seven women and seven men, plus expedition leader Christian Clot himself, 15 people in all, with the volunteers aged 27 to 50. They entered the cave in mid-March 2021 and stayed 40 days and nights, led by French-Swiss explorer Christian Clot, founder of the Human Adaptation Institute, on a project that cost around 1.2 million euros, or roughly $1.5 million.

Inside, there was no natural light at all. The temperature stayed around 50 degrees Fahrenheit, with humidity at 100 percent, and the group had zero contact with the outside world, no pandemic updates and no messages from family. With no clocks or watches, the volunteers relied on their own bodies to know when to sleep, eat and work, counting their days in sleep cycles rather than in hours.

The science behind the missing ten days
According to a 2026 peer-reviewed study in Chronobiology International titled ‘Circadian free-running and temporal organization during 40 days of human group isolation in a cave without external time cues,’ led by chronobiologist Benoit Mauvieux, who also helped monitor the group inside the cave, 13 of the 15 participants were tracked using wrist sensors, swallowable body-temperature capsules, and saliva samples for cortisol and melatonin. Before entering, their sleep-wake cycles averaged close to 24 hours. Once inside, that average stretched to roughly 31.6 hours per cycle, and the group completed only about 29 of these longer cycles across the full 40 real days. Their bodies were quietly running on longer days without anyone noticing, which likely explains why most undercounted how much real time had passed.

This lines up with earlier research. A 2020 case study, ‘Living Without Temporal Cues: A Case Study,’ in Frontiers in Physiology, followed one person living 10 days in a bunker with no direct human contact and no knowledge of actual clock time, no telephone, internet, television, radio, or natural light and found that the body's internal clock naturally drifts toward a longer-than-24-hour rhythm once outside time cues disappear, and a person's sense of what time it is drifts right along with it.

One runner's body clock went even further off script
Not everyone drifted by the same amount. According to the PBS NewsHour report, 37-year-old math teacher and sailing instructor Johan Francois ran roughly 10-kilometer circles inside the cave to stay in shape, and sometimes felt "visceral urges" to leave. When he came out, he guessed only 23 days had passed. Marina Lançon, a 33-year-old outdoor guide, described it differently, telling the AP that "it was like pressing pause," and that she wasn't in a hurry to check her phone once she was back outside.

Why this hits close to home for anyone who has lost a day working from home
For a generation that spent a chunk of 2020 and 2021 stuck indoors, unsure whether it was Tuesday or Saturday, this research might feel familiar. Remote work, blurred weekends and constant scrolling can do a smaller version of what that cave did: strip away the cues, like commutes, sunlight and fixed meeting times, that keep the body's internal clock anchored to a 24-hour day. It's a reminder that morning sunlight, regular mealtimes and a steady wake-up hour do more work than most people realize to keep a person's sense of time in check.

What researchers want to study next
Beyond curiosity, this research has practical uses. The Chronobiology International study notes that understanding how far human temporal organization can stretch before it breaks down could shape how schedules and lighting are designed for people in long-term isolation, such as submarine crews, polar researchers, and astronauts on extended space missions, where a normal day-night cycle doesn't exist.

Forty days without a single clock changed how fifteen people experienced time itself. It's a strange, slightly unsettling reminder that the sense of "now" most of us take for granted is more fragile, and far more dependent on the sun, than it feels.