More than 60 years ago, a French geologist spent two months of his life living underground in a cave.

By himself — with no light other than a bare bulb, no clock and very little contact with the outside world.

What Michel Siffre discovered in his 1962 experiment — about our biological rhythms, about loneliness, about time as we know it — informs scientists to this day. It may end up helping humans get to Mars.

It even plays a part in figuring out how to beat the wintertime blues.

Life in a cave

By going subterranean, what Siffre did was throw his regular body clock all out of whack. With no idea of sunrise or sunset, of night or day, he had no idea when to eat or sleep.

“My body chose by itself when to sleep and when to eat. That’s very important,” Siffre told Cabinet magazine in 2008. “We showed that my sleep/wake cycle was not twenty-four hours, like people have on the surface on the earth, but slightly longer — about twenty-four hours and thirty minutes. But the important thing is that we proved that there was an internal clock independent of the natural terrestrial day/night cycle.”

A few years later, Siffre talked a couple spelunkers into a similar experiment. Josie Laures spent 88 days alone in a cave, and Antoine Senni emerged from another cave 126 days later, as Julie Beck points out in The Atlantic. When they emerged, their body clocks — what scientists call their circadian rhythms — were as equally messed up. (You can learn more about Laures' experience in the video below.)

Siffre explains to Cabinet:

“I believe that when you are surrounded by night — the cave was completely dark, with just a light bulb — your memory does not capture the time. You forget. After one or two days, you don’t remember what you have done a day or two before. The only things that change are when you wake up and when you go to bed. Besides that, it’s entirely black. It’s like one long day.”

How whacked out were their clocks? Siffre ended his experiment on Sept. 14, as planned ahead of time. When he was told his time was up, he thought it was just Aug. 20. Laures and Senni reported similar miscalculations, as did a woman who spent 130 days in a New Mexico cave in 1989.

The reason for that lost time is easy to explain. At times, Senni slept up to 30 hours at a time or had waking period of up to 18 hours. Siffre didn’t go quite to that extreme in 1962 — a “48-hour cycle,” as he calls it — though he did 10 years later when he spent six months in a cave in Texas.

Without regular rhythms, a 24-hour day became meaningless. Of his six-month stint in the cave in Texas, Siffre told German news magazine Der Spiegel: "Physically it was not tiring, but mentally it was hell."

Shining a light on circadian rhythms

The experiments were first undertaken to study the effects of isolation and loneliness on humans, especially as they might pertain to putting an astronaut on the moon. The experiments make even more sense now as humans mull a trip to Mars, 14 million miles away.

But the cave-dwelling experiments have also helped us understand a much smaller endeavor: getting through the chilly months without succumbing to Seasonal Affective Disorder (SAD), a form of depression. SAD, also known as the wintertime blues, is thought to be brought on by a lack of sunlight and a drop in serotonin levels, according to the Mayo Clinic. Studies have shown what the cave experiments more than hinted at: that these light and body changes disrupt circadian rhythms.

It's clear you don't have to live in a cave to understand the importance of getting some sunlight in your day, but a little history certainly puts it into perspective.