00:00So, what's the longest you can survive without looking at your phone or watch to check the time?
00:06My personal record is about 13 minutes.
00:09But someone has no trouble living without knowing the time for 63 days.
00:15In the summer of 1962, a French chap named Michel Cifre took off his watch and went down into a cave in the French Alps.
00:23The then 23-year-old spent the next couple of months in complete isolation and without seeing the sun.
00:30And that's how he accidentally became the founder of a whole field of science called chronobiology, or the study of biological rhythms.
00:41Michel's original plan was to study an underground glacier he had discovered earlier.
00:46He wanted to spend 15 days underground, but then thought it wouldn't be enough to do serious research and decided to stay there for two months in the dark.
00:55It wasn't the comfiest of vacations, as his feet were always wet, and his body temperature was way lower than usual.
01:02He spent his free time reading, writing, thinking about his future, and studying his surroundings.
01:08He also slept well and ate when he felt like it.
01:11There was a team waiting at the entrance of the cave.
01:14They agreed the scientists would let them know when he woke up, ate, and before going to sleep.
01:20The team wasn't allowed to contact him, just to record when he was sending signals to make sure he was safe and sound and keep track of his life cycle.
01:29Cifre also informed them of his pulse and counted from 1 to 120, one digit per second.
01:35And that's how they found out his perception of time was seriously off.
01:39It took him 5 minutes to finish that task instead of the regular 2 minutes.
01:44When the team told him it was time to get out of the cave, he was confident it was only August, but it was already the 14th of September.
01:53Cifre thinks it had to do with an almost total absence of light.
01:57He had just a little lightbulb in the cave.
02:00It all felt like one long day to him.
02:02The most important result of his experiment was proving that people, just like animals, do have an internal clock that doesn't depend on the day and night cycles.
02:19Cifre didn't stop there and went on dozens of expeditions to spend some time inside caves.
02:25Ten years later, he started a six-month experiment in Texas.
02:30All this helped him to find that when people don't have time cues, they switch to a 48-hour cycle, not the 24-1 we're used to.
02:38It would be 36 hours of activity and 12 to 14 hours of sleep.
02:42NASA used the results of his experiments to help astronauts who had short-term memory issues after some time in isolation.
02:53Now, in 2021, a Spanish woman beat Cifre's cave record.
02:58Beatrice Flamini spent 500 days in a cave outside of Granada with basically no contact with the rest of the world.
03:06She insisted that the support crew wouldn't give her any news at all, even if something happened to her family members.
03:13The idea of this rough experiment was to test her own limits and also help scientists understand how staying in such extreme conditions affects the human mind and body.
03:23During the experiment called Time Cave, Flamini received food from a drop-off point and sent GoPro videos of herself to the support crew.
03:32They checked the footage to make sure she was physically and mentally fine.
03:37Flamini spent the entire time working out, reading books, painting, weaving, and making food.
03:43She didn't shower, but the support team took out her natural waste once every five days.
03:49At some point, flies filled the cave, but Flamini braved on and continued the experiment.
03:55Around day 300, she had to leave the cave and stay in a tent in isolation for about eight days
04:01because of a technical issue.
04:03When 500 days elapsed, the support crew found Flamini sleeping.
04:08She had lost track of time completely and said it always felt like 4 a.m. to her.
04:13She didn't want to leave as she hadn't finished the book she was reading.
04:17Although she started hearing things that weren't real and had a strong craving for roast chicken,
04:22the extreme athlete never even considered leaving the time cave earlier than planned.
04:27Researchers from several Spanish universities will analyze the data from this unusual experiment.
04:36Intrigate biological rhythms govern our lives, from the smallest cellular processes to the functioning of the entire body.
04:44These rhythms regulate various aspects of your physiology, including sleep patterns, body temperature,
04:51hormonal balance, metabolism, and cardiovascular activity.
04:55Many diseases get stronger during nighttime or in the morning.
04:59Shift work disrupts the balance between internal rhythms and external time, so scientists blame it for diseases.
05:06Jet lag is a good example of how your internal clock gets out of whack because you move to a different time zone.
05:13But light, especially when you catch it at certain times, can help reset your body clock.
05:18When you see light at the end of the night, it nudges your clock forward, and early evening light can push it back a bit.
05:26It takes about a day for your body to catch up with each hour of time difference.
05:30And since our natural body clock ticks around 24.2 hours, it's easier for us to adjust to longer days out west than shorter ones out east.
05:40That's why when athletes or researchers live underground for a while, they end up feeling like they're in a time warp.
05:49Back in the 70s, scientists found the circadian clock mechanism in a fruit fly.
05:54It has to do with different genes.
05:57One gene revs up the other, which then puts the brakes on the first gene, creating a swinging rhythm.
06:02During the day, when the sun's up, a photoreceptor tells certain parts of the loop to relax.
06:09There's a whole complex network of molecules and neurons making sure everything ticks just right.
06:15Every living thing's got its own circadian clock, with its own set of clock genes doing the work.
06:21Besides light, other stuff, like temperature and food, helps sync up an organism's clock with the outside world.
06:27Now, if you're feeling adventurous and want to try cave isolation yourself, how about the deepest one on our planet, Varyovkina Cave?
06:39It only got this title in early 2018.
06:42It was explored step by step, deeper and deeper.
06:45The first expedition to this cave in the West Caucasus only explored around 5% of it, but it was already nearly 400 feet.
06:53It was obvious there was much more to this cave, with a small entrance diving into the limestone of Earth's crust.
07:00Expeditions from all over the world went back there until they finally reached the bottom.
07:05The world's second, third, and fourth deepest caves are all nearby, hiding deep in the mountains of the Arabica Massive.
07:13It is one of the largest karst massives on Earth.
07:16But it's still not well explored because it's really tricky to get to this area.
07:21Only accessible up to four months of the year because of weather conditions.
07:26Going all the way down there is really tricky and takes several days.
07:30There are underwater waterfalls with near-freezing water, flooding, obstructed entrances, and other surprise dangers.
07:39So, if you aren't ready for the deepest of them, you could go with the longest one.
07:43Mammoth Cave in Kentucky has over 400 miles of limestone labyrinths, already explored, and probably around 600 miles still to discover.
07:54Over a half a million people visit the cave every year as part of organized tours.
07:59You can choose between a classical and more extreme version, learn about the cave's cultural history, or attend a concert within its stone walls.
08:07You can also enjoy meeting over 100 wildlife species living there.
08:12One of the most famous and unusual of them is the eyeless cavefish.
08:17It has adapted to the environment without light by no longer growing eyes.
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