Five Experiments That Took Away the Clock

In 1938, two men went into Mammoth Cave, Kentucky, and stayed 32 days. They lived in a place with no dawn, no dusk and no weather. They wanted to know if a person could be talked out of the 24-hour day.

Since then, a small group of researchers have repeated the trick in caves, bunkers, labs and tents. When people live without clocks or sunlight, the human body clock keeps going on its own, but it runs a little longer than 24 hours and drifts later each day. Daylight, and in some people a schedule, pulls it back.

Here are five experiments that show it, each with a different lesson.

1. Kentucky, 1938: a 28-hour day

Nathaniel Kleitman was a sleep researcher at the University of Chicago. In 1938 he took a graduate student, Bruce Richardson, into Mammoth Cave for 32 days. They used electric light and camping lamps that they switched on and off themselves, and they followed a 28-hour day set by an alarm clock. Four extra hours, every day.

The result was split. Richardson’s body temperature cycle stretched toward 28 hours. He slept well on the long day. Kleitman was 43, and his temperature rhythm stayed close to 24 hours. He got tired at the old time and woke at the old time, no matter what the schedule said.

Kleitman concluded that the sleep and wake rhythm was partly habit and could be moved, but that people differ in how far. Age was his suspect. Two people is not a sample, so treat the age idea as a guess he raised, not a result. Kleitman’s better-known work came fifteen years later, when his student Eugene Aserinsky’s eye-movement recordings led to the 1953 REM discovery.

What it changed for alarm-bound life: a day that is not 24 hours is hard for most bodies to follow. Your alarm works on a clock that does not fit your body perfectly, and it only works because the body is flexible enough.

2. The Alps, 1962: a month that went missing

On 16 July 1962, Michel Siffre, a 23-year-old French geologist, went down into Scarasson, an ice cave in the Ligurian Alps about 130 metres below the surface. He had no clock and no calendar. A phone line ran one way: he could call up to his team, and they were told never to tell him the time or the date.

He ate when hungry and slept when tired. He called the surface each time he woke, ate or went to bed, so the team could log it against a clock he could not see.

He came out on 14 September, after about two months underground. By his count it was only 20 August, nearly four weeks behind the real date. His sleep and waking cycle ran about 24.5 hours, and his count of days fell behind the surface count.

What it changed: the first person known to live this way showed that the body keeps its own beat, and that the mind’s sense of “a day” is not reliable. Feeling like you slept enough is not the same as sleeping a fixed span. Without a clock outside you, you cannot tell.

3. Texas, 1972: a clock that came apart

Siffre went back in 1972. On 14 February he entered Midnight Cave, near Del Rio, Texas, and stayed 205 days. He ran the same rules, with a team at the surface logging his calls.

The long stay showed something the first one could not. In two stretches his cycle approached 48 hours, though not regularly. The rhythm was not steady. When the team ended the experiment in September, he thought it was still mid-August.

What it changed: the clock is not a metronome. When you give it nothing to hold on to, it can wander for days, then settle again. That is why a plain, repeated morning cue matters. It is not there to wake you. It stops the wandering. The zeitgeber post covers which cues do that job.

4. Boston, 1999: the number 24.18

For decades, people repeated that the human clock runs 25 hours. That figure came from isolation studies in which volunteers could turn lights on and off as they liked, and the light itself pushed the clock around.

Charles Czeisler and colleagues at Harvard fixed this. In their 1999 paper in Science, they used a method called forced desynchrony. Volunteers lived on a 28-hour day. That day is too long for the clock to follow, so it ticked at its own pace while sleep and waking rotated around it. Light could not sneak in as a hidden cue.

Their measured period averaged 24.18 hours in both a young and an older group. In the older group, the reading was 24.18 with a spread of just 0.04 hours. Do the arithmetic. 24.18 minus 24 is 0.18 hours, about 11 minutes. Left alone, that is how much later your clock would run each day. In a week, that is a bit over an hour.

What it changed: the pull of your morning alarm is small. The drift is 11 minutes a day, and a steady cue can absorb that. Late weekends add up the same way, as social jet lag shows.

5. Colorado, 2013: a week with no lamps

All the cave studies took light away. Kenneth Wright and Andrew McHill at the University of Colorado Boulder did the reverse. They gave people only the sun.

In 2013, they published in Current Biology a study of eight adults. After a week of normal life with work, school and electric light, the group’s melatonin timing was measured. Then they camped for a week in the Rocky Mountains of Colorado, in summer, with only sunlight and campfire for light.

After the week, melatonin began to rise about two hours earlier than before. Bodies that had lived late under lamps moved to match the sun.

What it changed: the “night owl” in a modern house may owe some of it to the lamps. The study did not track people after they went home, so it cannot say how long the shift lasts. Part of a late clock is the light you live in.

What the five add up to

Kleitman showed that people differ in how far they bend. Siffre showed that you cannot trust your own count of days, and that the clock can wander. Czeisler measured the drift, and Wright and McHill showed that light can move the clock. Your alarm is one more cue. It is a blunt one, as the history of the alarm clock shows.

What a body clock does without clocks, in one paragraph

When people are cut off from clocks and sunlight, the human circadian clock does not stop. Studies from Siffre’s 1962 cave stay to Czeisler’s 1999 Science paper put its natural period near 24.2 to 24.5 hours, so without a daily light cue it drifts later by roughly 11 to 30 minutes a day. People in isolation also lose track of the date, because the sense of elapsed time follows the body’s long day, not the sun’s.

Frequently asked questions about living without clocks

Did Michel Siffre keep doing cave experiments after 1962?

Yes. He went back underground for more than six months in Texas in 1972, with NASA interest, and continued time-isolation work for decades. He died in 2024 at age 85. The 2017 Nobel Prize in Physiology or Medicine, awarded to Jeffrey Hall, Michael Rosbash and Michael Young, went to the genes that run the clock inside cells.

How did Czeisler’s team get the clock to show itself?

They put volunteers on a 28-hour day: bedtime moved four hours later each day for about three and a half weeks. The body clock cannot follow a day that long, so it ticks along at its own pace while sleep and wake rotate around it. That lets researchers pull the two apart and measure the clock alone.

Does camping fix a late body clock for good?

The 2013 study measured the clock after one week outdoors and did not follow people home. It shows the clock can move earlier when light follows the sun. It does not show how long the shift lasts once electric light comes back.