PARIS -- It is just a short walk, or an even shorter drive if you are a top player, to get from Roland Garros to the French Tennis Federation's National Training Centre, a state-of-the-art facility that is available to French players throughout the year, free of charge. With six indoor courts and various gyms, it has everything elite players need to train.

But for two weeks a year, during the French Open, it becomes home to many top players, who come for another reason: its recovery facilities. And one kind of recovery, in particular -- cryotherapy.

Cryotherapy involves immersing the body in air frozen to temperatures that can be lower than minus-150 degrees Fahrenheit for a short spell of time. The idea is the extreme cold helps the body recover faster, while also reducing injury, raising energy levels and even improving sleep. While many athletes have long used ice baths, cryotherapy rooms or machines have become the preferred choice of many top sports stars, including soccer player Cristiano Ronaldo.

All players in the main draw can use the cryotherapy rooms for free, and Rafael Nadal, Stan Wawrinka, Alexander Zverev and many others have been frequent visitors to the rooms in the past fortnight. Go up one floor from the entrance and turn a corner, and you are confronted by two small rooms, marginally bigger than your average shower cubicle. One is set at minus-60 degrees Celsius (minus-76 Fahrenheit) and the other at minus-110 degrees Celsius (minus-166 Fahrenheit).

Once there, the fun begins. Stripping down to their underwear, players then put on a mask to cover their mouths, extensive tape around their heads to cover the ears, big gloves, socks and slippers, before stepping into the first chamber.

"The protocol is they stay 20 seconds in the first chamber at minus-60, then they enter the other chamber at minus-110, for three minutes normally," said Mathilde Poignard, a PhD student in physiology and sports science who looks after the whole-body cryotherapy chambers and is writing a study examining the benefits.

The first time an athlete uses cryotherapy, it can be difficult if they are not warned about the effect of the cold. "The air comes at you -- it's cold, but there is also maybe difficulty to breathe the first time because you have to breathe slowly [not to panic]," Poignard said. "We try to make them aware that they have to breathe slowly and be calm. If we don't tell people this the first time, maybe they will be stressed when they enter the chamber."

Novak Djokovic, Feliciano Lopez, Grigor Dimitrov and Eugenie Bouchard are among those known to have tried cryotherapy, and it seems tennis players in general are buying into the idea. Poignard said "many top players" have been using the chambers during the tournament. Nadal is a regular, including a session after his straight-sets semifinal win against Roger Federer. Wawrinka, who, like Nadal, has used cryotherapy for several years, posted a photo on his Twitter feed of him coming out of the chamber.

There is an alternative version of cryotherapy, where players stand inside a machine, with their head exposed at the top, away from the cold, as used by players at the Mutua Madrid Masters. At the French facility, though, the rooms allow complete immersion in the cold. Players can stand still or move around, and if they feel discomfort, they can leave.

"We can also talk to them every minute to tell them the time," Poignard said. "There is also a door if they need to come out. After, they have to stay calm, normally for 30 minutes, but sometimes they leave quickly after. They may have a cold shower, but not a warm shower, because if you do a warm shower, you stop all the benefits of the cold."

Zverev was one of the players who used it for the first time this year. "We are in there for three minutes, and it really, really hurts," he told German reporters this week. "After those three minutes, you are ready for a hot shower."

Rafael Nadal was one of the top players at the French Open to regularly use cryotherapy, including after his straight-sets semifinal win against Roger Federer. Clive Brunskill/Getty Images

Zverev, who was convinced to try it by another player, said it had one particular benefit. "You do really sleep very well," he said.

Cryotherapy is used in many sports and also has become the recovery of choice for Hollywood celebrities, but Poignard said the jury is still out as to just how beneficial it really is.

"The main benefit is for the recovery and muscle soreness," she said. "But at the moment, there is no proof of other benefits, other than muscle soreness and the perception of muscle soreness. There are several studies about it, but some studies are contradictory and maybe depending on the context. ... There is a different exercise, and it's difficult to compare and make one conclusion."