LA Fitness -- Turns Into A Prison For One Dude

Sitting in a hot sauna after a hard workout is supposed to be relaxing ... but for Nicholas Clayton it turned out to be lot more stress.

The gym staff locked up at 5 PM -- seems pretty early -- and Nicholas obviously thought so too because he was still baking in the sauna when they closed down the gym for the night. He documented the experience on video and posted on Facebook.

Good thing for him, he didn't have to curl up on a yoga mat and wait it out til they opened the next morning ... he finally got in touch with gym management and they were able to get him out.