However, one period of the training that lasts just 36 hours of the six-month selection process stands out above all as the most agonising and cruel.
That's when the potential recruits are trained to withstand torture and interrogation with tactics which include being put into a stress position and blasted with white noise.
Author and former SAS soldier Chris Ryan believes that to be the most difficult part of the selection process.
He would know, too. Not only was Ryan an SAS soldier, he also trained and selected recruits by observing this process.
Speaking to LADbible, Ryan explained how the process is supposed to train potential SAS recruits for 'every aspect of survival'.
After spending two weeks living for themselves with little to no food, shelter or provisions, the recruits are then taken out for 36 hours of 'tactical questioning'.
Ryan said: "The SAS have a dispensation where we can subject our students to 36 hours of interrogation. It's probably - in terms of the six months of selection - the 36 hours are the hardest you'll ever go through in a concentrated period.
"You're isolated, you are tactically questions - I'm being careful with my words here - you are kept in an uncomfortable position for that period of time with no sleep, and you are resisting giving any information that you would be holding out.
"What we've realised now in this modern world is that the information that a special forces soldier has on him, he has to hold it for 24 hours."
"After that, within that 24 hour period, that information will be rendered useless.
"It's to give information to potential soldiers on how to resist giving that information up."
The methods used are meant to expose recruits to some of the techniques that they could be subjected to if captured on a mission. The aim is to replicate this closely to see how the recruits stand up to it, and whether they crack and give up information.
Ryan explained: "You're put in a pen and 'processed', they'll strip you naked and you'll get a thorough examination to make sure you don't have anything hidden on you, so you're sterile as a human being.
"You end up blindfolded at this point and you don't hear any other noise. You're given a set of overalls and taken into a room that has large speakers on the wall that are pumping out white noise.
"It actually works against the frequencies of your brain.
"You're put in a stress position, which is your legs spread out and your arms spread out and you're placed against a wall at an angle.
"The pressure is on your neck, your arms, your legs.
"Then, just as you're starting to collapse, they'll sit you down on the floor and cross your legs, then place your arms on your head but not interlinking your fingers, just placed.
"Your arms and everything ache.
"With the static going on, it starts to affect your brain. You'll start hearing noises and voices, your eyes will start seeing colours and various other images."
You might think this is where it ends, but you'd be wrong.
"[Then] you're taken into another room and interrogated," Ryan added.
"You will have a guy who looks like a normal clean cut guy, he's going to be Mr Friendly. If you sign this piece of paper or give him some information you can have a piece of chocolate or a cup of tea.
"There's another guy who looks a right nasty b***ard. He's going to threaten your life, probably be a bit physical with you.
"You'll have another guy who will be very repetitive and you'll think in your mind that he's a dithering idiot."
However, that's still not the worst of it.
"The ones you've got to watch for are the female interrogators.
"Most of them, when you go in, they'll get you to take your coveralls off, and obviously it's freezing cold. Things between your legs disappear.
"What she'll do is start pulling the p*** out of your body.
"On my selection, there were six of the biggest lads, all hard as nail and Falklands veterans, or seen action in other places - they all cracked under a female interrogator.
"They'd just blown four and a half months hard work, they were taken off selection.
"It's keeping your mind sharp and conducting yourself correctly, which you've been taught to do during combat survival."
Ryan was speaking to LADbible ahead of the release of his new book, Manhunter.
Manhunter is the first novel in a new series from the One That Got Away author, and the first to feature Joe Bowman, an 'ex-SAS hero battling opioid addiction and his own inner demons'.
The synopsis reads: "Following a botched operation in Afghanistan, Bowman is recruited into 'The Cell', a covert unit within the SAS working alongside the police - a unit so secretive most of the other guys in the regiment don't even know it exists."
Manhunter will be available in all good booksellers from 27 May.
Featured Image Credit: Mick Kavanagh/PA
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