The same method is still used by long distance runners. Nowadays, they sleep in a tent (indoors) where the oxygen concentration is artificially reduced to simul

The same method is still used by long distance runners. Nowadays, they sleep in a tent (indoors) where the oxygen concentration is artificially reduced to simulate sleeping at a high altitude: https://en.m.wikipedia.org/wiki/Altitude_tent https://en.m.wikipedia.org/wiki/Altitude_tent