Man spent months alone underground in a cave and it had an unbelievable effect on his body clock

One scientist went to extreme lengths to test how the absence of daylight affects humans.

In 1962, researcher Michel Siffre isolated himself in a cave to study how humans experience time when there are no external cues.

For two months, the scientist experienced life with no access to a clock, calendar, or the Sun.

Our circadian rhythms, or internal clocks, are roughly 24 hours long and are typically synchronised with the Earth's day-night cycle.

They are regulated by several factors (or zeitgebers), the main ones being daylight and temperature.

Without them, our 24-hour cycle actually shifts to a 25-hour pattern, so an hour longer than what people would experience normally.

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Reflecting on his experiment, the French explorer explained: "This idea came to me - this idea that became the idea of my life. I decided to live like an animal, without a watch, in the dark, without knowing the time.

"Instead of studying caves, you ended up studying time. Yes, I invented a simple scientific protocol.

"I put a team at the entrance of the cave. I decided I would call them when I woke up, when I ate, and just before I went to sleep. My team didn’t have the right to call me, so that I wouldn’t have any idea what time it was on the outside."

Without light to guide him, Siffre experienced time in a radically different way.

To keep track, he monitored his pulse and performed 'psychological tests.'

He discovered that what felt like two minutes to him was actually five minutes in real time.

Essentially, he was experiencing everything twice as slow.

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"I had to count from 1 to 120, at the rate of one digit per second," the 85-year-old described.

"With that test we made a great discovery: it took me five minutes to count to 120. In other words, I psychologically experienced five real minutes as though they were two."

In fact, by the time Siffre emerged from the cave, he thought he had a month left in the experiment, completely unaware of how much time had actually passed.

And you may wonder how Siffre kept himself occupied in the silence and darkness?

Well, unlike many of us who would've probably gone a bit crazy left with our minds for so long, the then-23-year-old spent his time productively by 'reading, writing and doing research', while also daydreaming about his future often.

Siffre's work has helped found the field of human chronobiology.