A FIFPRO report has found that top-level footballers are under increasing "mental stress" due to fixture workload, with Erling Haaland's "machine-like" start to the season with Manchester City proving the need for a prolonged period of rest.

Haaland, 24, has started the Premier League season with seven goals in three games for Pep Guardiola's team after having a full summer break due to Norway's failure to qualify for Euro 2024.

On the flip side, however, both Jude Bellingham (Real Madrid) and Phil Foden (City), have already missed club games through injury or fatigue this season after their Euro 2024 campaign with England lasted until the final in Berlin on July 14.

Haaland, Bellingham and Foden are set to play in the FIFA Club World Cup at the end of this season -- a 32-game tournament in the United States which ends on Jul 13, 2025 -- which will make it the longest club season in history.

The FIFPRO study has found that elite footballers now have as little as 12% of the calendar year off duty -- less than one a day a week -- with former City forward Julián Álvarez, now with Atlético Madrid, emerging as the busiest player in the world last season with 75 appearances and 83 matchday squad inclusions for club and country.

And after surveying Premier League stars, Maheta Molango, the CEO of England's Professional Footballers' Association (PFA), said that Haaland's start to this season shows just why players need more rest.

"I found it astonishing the difference there was in terms of the feedback from the people who had a proper holiday and those who did not have a proper holiday," Molango said. "And the body language and the words they choose in talking to us was such a big difference.

"For us in England, we have a very clear example in Erling Haaland. It was very nice to go to a dressing room and hear someone [Haaland] say to you: 'I was missing being back, I was missing being able to train again and I'm pumped up, I'm motivated, I'm here.'

"That was in preseason and now you see the results. I mean he's being back to the machine that we saw when he first joined us in England and I think it's probably something very similar to Mohamed Salah -- he also had the proper rest and you can see that he is again the best version of Mo.

"It was very telling for myself to see what the feedback was from the people with the proper rest, how they were looking forward to be back and how they're performing right now from the feedback of the people who have not had the rest and you can see how they look shattered. They looked tired even before they started the season, which for us is very worrying."

While the number of games being played by top players is increasing, FIFPRO said it believes the true measure of a footballer's activity should now be measured in squad appearances -- games played and those travelled to as squad members -- due to the demands places on players by mental strain as well as physical demands.

"It's a real progression for us to look at squad inclusion rather than just match minutes," Darren Burgess, FIFPRO adviser and former Liverpool and Arsenal performance coach said. "Because previously, if you look at Phil Foden coming off the bench for seven minutes, you think, well that's not so much.

"But the game might be an 8 p.m. European game or a midweek cup fixture. He's had to travel, potentially internationally, for that. Most of the time when you travel for European games you either stay in that country overnight and get back in the middle of the day the next day or even more commonly you travel, because the players want to sleep in their own bed and frequently arrive plus three or 4 a.m. in the morning.

"So for seven minutes of match play that player, in this case Phil Foden, would experience the total stress involved with the travel, no sleep, the impact of having to train the next day.

"So it really is a much better and more holistic metric rather than just the match minutes because the stress on the body is what we know, but what the research is now telling us, because we're able to measure it, is that mental stress equals physical stress and we're able to look at the brain under functional MRI and know this.

"These players are clearly going under a large degree of mental stress watching the game, wondering whether they're going to come on or off the bench. Poor sleep equals poor performance and injury risk also. So it's a far more accurate metric."

FIFPRO and players' unions from the major leagues in Europe launched a legal action against FIFA in June, accusing the world governing body of an "abuse of its dominance" in the game.

The case is ongoing and could yet impact the Club World Cup, with PFA CEO Molango adding that the number of games being imposed on top clubs makes this a "defining" season for football's future.

"This season will be the defining season," Molango said. "This season will be the season where this just does not make any sense, no matter how you look at it and that's why it's so important that we talk about it this season.

"For a number of years and months, we have been warning about what would come and I think this year, we're going to have a few very telling examples of what should not have happened in football."