Image copyright Getty Images Image caption The final of the International 2018 in Vancouver, where teams competed in the game Dota 2

The numbers are dizzying: more than $33m (£27m) in prize money at an upcoming tournament. Competitors from around the world, many still in their teens. And the pressure that comes from knowing that millions of viewers are watching your every move.

The world of e-sports is not without its detractors - including Prince Harry, who said it's "more addictive than drugs" - but for young, talented players, it's a chance to make it big in an almost billion-dollar industry.

Kyle Giersdorf, aka Bugha, is a 16-year-old high school student from the small town of Pottsgrove in Pennsylvania - and as of last month, he is a multimillionaire.

In July Kyle won the Fortnite World Cup, which had a record-breaking $30m (£24.6m) prize pool. His first place prize, of $3m, was also the highest ever individual win for an e-sports event.

The figures are overwhelming - but those records are set to be dwarfed this week. The International (TI), a tournament of the game Dota 2 in Shanghai from 20 to 25 August, has already surpassed Fortnite's prize pool. At the time of writing, the pool is more than $33m.

But it's not easy money to make.

'A test like no other'

Anucha Jirawong, also known as Jabz, is a 20-year-old professional gamer from Bangkok. His team, Fnatic, are among the elite few now in training for TI.

Jabz tells BBC News that he's been gaming since he was 13. Even when it was just a hobby, he was playing for about seven hours on weekdays, around school, and 13 hours on the weekends.

Now that he's gone professional, his schedule is even more intense.

"We typically wake up around 10:00, have a shower and lunch, and we jump into a [team] meeting at 10:30. That's followed by three training matches against teams in the circuit and a lot of discussions," he explains.

"We usually have our dinner at 18:00, sandwiched between training matches and a final discussion at around 22:00. Then I play some public-ranked matches to hone my skills further, until 01:00."

Damien Chok, a 26-year-old Australian player whose Dota 2 name is Kpii, has a similar schedule. He tells the BBC that his team - Mineski - practises together for about eight hours, after which he spends another few hours playing solo.

Image copyright Getty Images Image caption Kyle Giersdorf, aka Bugha, won $3m in the Fortnight World Cup last month

This is far from unusual. Jack Chen, who's managing a team at this year's TI, says most competitors "often [play] within the range of eight to 16 hours each day".

"We're going to be competing for the biggest prize in e-sports at the event that has been its Everest," he tells the BBC. "This is the event that defines Dota players' legacies and careers, puts them forever in fans' hearts and minds, tests them like no other against the best in the world."

'Every mistake is toxically scrutinised'

Despite the competition being "life-changing", there are downsides, Mr Chen adds. "Competition is fierce, and at some point that means things cross the boundary from play into work. Everyone has to sacrifice and do things they otherwise might not to get an edge and improve against an ever-evolving field."

Add to that an overwhelming level of sudden fame and recognition - as well as the public scrutiny that comes with it.

Tyler Erzberger, an e-sports journalist for US sports broadcaster ESPN, wrote last year of the intense criticism that players face, of "playing on stage and having every mistake toxically scrutinised on Reddit, Twitter and other online forums".

Image copyright Getty Images Image caption Dota 2 fans dressed up in cosplay in Seattle, Washington, at The International 2014

He also described a "concerning scene" at the Evolution Championship Series (Evo) last August, when top competitor Justin "Plup" McGrath had a panic attack on stage.

"There is no other sport in the world in which one day you can be a teenager playing a game by yourself, and the next day, because someone scouted you from your online account, you're thrown on to a stage for millions to criticise," he added. "There's little to no assimilation period. There's no road map for how to deal with the criticism."

There have, as yet, been no major studies into the mental health risks of professional gaming. However, many experts have anecdotally linked it to conditions such as anxiety and burn-out. Most pro gamers retire before they even hit their mid-20s.

'No-one gets better at 3am'

Dr Doug Gardner, a sports psychologist in California, has worked with Dota 2 teams training for TI. He tells the BBC that from what he's observed, these intense periods of gameplay can actually be counter-productive - both for the quality of the game, and for players' health.

"People, whether it's in Dota 2 or other environments, think that more is better," he says. "And I do think that you get to a point where there's a point of diminishing returns. You're cognitively, physically, emotionally, mentally [affected]."

When Dr Gardner first started working with a Dota 2 team, in the run up to last year's TI in Vancouver, Canada, he says the team members were fighting with each other, they were sluggish, and they were up until the early hours of the morning "mindlessly playing games". Their day was essentially: eat, sleep, game, repeat.

"I personally don't think any person is going to get better at their craft at two, or three or four in the morning," he says.

Image copyright Getty Images Image caption Training for TI is intense - and the stakes are high

Seeing the young players developing potentially damaging habits, he set up a new routine for them to follow - inspired by his work with professional American football and baseball players.

Rather than getting up at lunchtime and immediately playing Dota 2, the teammates had to wake up at 09:30, get ready and go to the gym for 10:00. They would then play Dota 2 for six hours, from 12:00 to 18:00. The evenings were suddenly free for them to spend as they wished.

What he found was that not only were the teammates happier, healthier and working together more harmoniously, their performance in the actual game improved too.

Gaming has its controversies - but Dr Gardner hopes that, given it's a relatively young sport, teams will start focusing on "quality practice, not quantity" in the future.