The length of modern online tournaments is definitely challenging. That's why good poker players work not only on their game, but on their performance too. They pay a lot of attention to pre-game preparation, take quality breaks, have good snacks, etc. And work on their concentration and awareness long term - a good history of quality sleep, physical activity, sports, meditation, other interests and hobbies, good work/rest ratio etc. Being a poker player has become as being an athlete.