Well, cave really. 
I had not heard of this project before this article. I’m not sure how much actual scientific value there is in this kind of thing (more than none, certainly), but I still think it’s very cool.
As I await the remaining six days to full immunity, I find myself feeling like I, too, am emerging from a cave, yet I know my experience was much less extreme. Did they have no light at all? What projects could they work on in total darkness?
No sunlight, the article says.
Finding additional information on other stories where this has been done in the past was one of those Cruelty to Google Users things, but here’s a story about a man and a woman who did this in 1965.
What this did to the woman’s menstrual cycle is not addressed in the story. This story does say what happened to another woman who did a similar experiment; sadly, she committed suicide in 1990 and this is her obituary announcement.
I had previously heard of Michel Siffre, referenced in the above articles.
In the OP I wrote:
From Mr. Siffre’s Wikipedia page:
So there is definitely more balue than “none” to this type of activity; that totally fucking rocks.
In the 1800’s a group of TB patients spent about 5 months in Mammoth Cave, Kentucky, to see if it would help with their condition. Three died.
They had lights, but they had no sunlight or clocks or calendars. Results varied by participant but many were surprised when the cave door opened because they didn’t realize it had been 40 full days. Several thought they had 10 days to go yet.
There were those 33 miners in Chile who were trapped in a cave-in for 69 days until they were rescued. When they began pulling them out of the mine one at a time, it was live-streamed. I watched them pull the first few guys out.
IIRC, they pulled them out during the night so they wouldn’t suddenly be exposed to sunlight so fast, and even at night they made them wear dark goggles, so they could be exposed to light gradually.