The term “self-isolation” has become a disturbingly common part of our everyday lexicon over the past year, although a group of volunteers has taken the concept to another level by spending 40 days in a cave with no natural light and no contact with the outside world. The 15 temporary troglodytes entered the Lombrines cave in France on March 14 as part of the Deep Time research project, which sought to examine the effects of extreme isolation on human physiology and psychology.

Exiting the cave wearing sunglasses in order to help their eyes readjust to daylight, the group seemed in high spirits. Unsurprisingly, however, they showed signs of having lost their sense of time, with experiment leader Christian Clot explaining that most participants believed they had only been underground for a month.

“And here we are! We just left after 40 days … For us it was a real surprise,” Clot told the Associated Press awaiting the group’s emergence from the cavern. “In our heads, we had walked into the cave 30 days ago.”

While inside the cave, the group slept in tents and used pedal bikes to generate electricity in order to power their torches. They had no phones or clocks, and had to draw water from a well 45 meters (146 feet) underground.

Completely cut off from the Sun and with no way of telling the time, the group had to rely on their circadian rhythms – or body clocks – in order to decide when to eat and sleep. Meanwhile, researchers used an array of sensors to monitor the team’s body temperatures, sleep patterns, social interactions, and behavioral and cognitive responses to their new environment. Volunteers' brain activity was also recorded before and after entering the cave.

According to the Deep Time website, the project aims to provide important information about the adaptability of humans to novel and extreme living conditions. This may include such scenarios as “lunar bases… [and] other extra-terrestrial territories,” as well as submarines or other underground habitats that may need to be occupied “in the event of severe climatic disturbances.”

Explaining the motivation behind the project, Clot stated that "our future as humans on this planet will evolve," before adding that "we must learn to better understand how our brains are capable of finding new solutions, whatever the situation."

While the project organizers claim that theirs is the first study to scientifically measure the effects of such extreme isolation, the 15 volunteers are far from the first people to live in a cave for science. Back in 1962, a Frenchman named Michel Siffre became the first to propose the existence of a biological clock after recording his sleep cycles during a 63-day stay in a cave in the Alps.

Siffre’s discovery sparked the emergence of chronobiology as a field of scientific investigation, although his future research led him to some dark places – literally and emotionally. In 1972, for instance, he spent six months living alone in a cave in Texas, during which time his sleep cycles became highly unstable and he experienced severe depression. He even managed to electrocute his brain after a lightning storm above ground messed with the electrodes that were attached to his head in order to monitor his neural activity.

Fortunately, the Deep Time volunteers came through their 40-day experiment unscarred, with two-thirds of participants saying they would have liked to have spent a little longer in the cave.