RTÉ's Ultimate Hell Week is a high-octane series that pushes people to their mental and physical limits, challenging participants to overcome cold-water events, height tests, and claustrophobic trials.

Based on the actual exercises currently used the on the ARW (Irelands Special Forces) selection course, the instructors have designed a series of obstacles that will test recruits physical, emotional and psychological resilience.

Designed to break all but the toughest of individuals, the failure rate is upwards of 90 per cent, with only the toughest, most mentally robust candidates passing selection.

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So, my first question to John Sharpson is obvious: why? Why on earth would anybody sign themselves up for such an ordeal? Well, ironically enough, the TV teacher says it's because he didn't do his homework.

"To be totally honest with you, I had no idea what it was when I signed up," he laughs. "I was sitting on the couch, minding my own business, when a fella I worked with before called Pat rang me."

When asked by his colleague to go on Ultimate Hell Week: The Professionals, John said: "Yeah, why not?" before heading to YouTube to do some digging. It wasn't long before the reality of what he had signed up for started to kick in.

"They send hiking boots out to us and say that the most important thing is to get used to them. Then they gave us pointers like telling us to get acclimatised to cold water, but not to try and deprive ourselves of sleep because we would need to be rested when we got down there. They also told us to get out walking with weights on our back."

"No amount of training you do can prepare you for what's going to hit you when you get down to hell week," he insists. "You can be as fit and as strong as you want, it's still going to absolutely hit you mentally and physically."

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Adding to the challenge, Sharpson discovered that he would be taking part alongside 17 other well known personalities from the world of sports and entertainment - including a number of high-profile athletes.

"I don't know if you've seen Peter Stringer but the man is an animal, he looks like he's made out of metal," he jokes. "Stephanie Roche is a professional athlete; you've got Andrew Trimble; Valerie Mulcahy has played inter-county football for Cork; Niamh Cullen is one of the fittest people I've ever met in my life."

"I'm a teacher," he adds, laughing.

Each recruit is vetted ahead of filming to ensure that they are of sound body and mind, however, John says that sleep deprivation will play havoc on even the steadiest of heads.

"Because you're not sleeping, you start to go a bit funny," he says. "Someone is shouting, you're not sure if you've packed your bag, everybody is moving - there's about a thousand and one things going through your brain and your brain hasn't had enough time to even process what's happening."

"You're in a constant state of panic and it's all being filmed," he adds.

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Failure to meet the required standards of the show means that the celebs will be asked to leave the course. For those who decide they cannot go on, they have to D.O. (drop out) by handing in their number to the DS.

To stop the recruits from dropping out at the first hurdle, the Hell Week team have upped the stakes by linking each celebrity to the charity of their choice. The longer they last, the more they will raise for their worthy cause. No pressure.

"The charity is Zamda and it stands for Zambia Direct Aid. It's a school principle, and an old hurling manager of mine, who went over to Zambia as a teacher and saw how bad things were for kids over there so he decided to go over and set up a charity."

"I've been over twice myself - once building and once teaching and structuring the curriculum. They basically take homeless kids in and provide themselves with clothes, education, and fascilities to wash. If they don't have anywhere to go, they try and board them while they find some family members for them. It's incredible work."

Despite the extreme trials, tribulations and sleep deprivation, Sharpson says he's still speaking with his good colleague Pat, and says the experience was worth it for the memories made.

"It's such a great opportunity because you're never going to get that challenge unless you're in the army. For a primary school teacher from Dublin to be able to so something like that was a great experience."

"Did I love it? No, not all the time. Did I cry? Yes, a lot," he laughs, "but am I glad I did it? Yes, 100 per cent."

Watch Ultimate Hell Week: The Professionals on Wednesdays at 9:35pm on RTÉ One