According to the Washington Post 16-time Olympic medalist Michael Phelps said Wednesday he had been sleeping “at 8,000 feet every night” for almost a year, after noticing he bounced back from workouts better when he trained at altitude, and therefore trying a device that simulates that.
“We’ve been able to realize after going to Colorado Springs so many times that it is something that helps me recover,” Phelps said. “That’s something that is so important to me now being older. I don’t recover as fast as I used to.”
They are for some reason calling it a hyperbaric chamber, which as far as I know would be a chamber with higher air pressure than what’s normal at sea level, and therefore the exact opposite of altitude. But there you go.