I've played in an intense chess tournament where both players have two hours on the clock, meaning a game can take up to 4 hours plus a few extra seconds per ea

I've played in an intense chess tournament where both players have two hours on the clock, meaning a game can take up to 4 hours plus a few extra seconds per each move. After some of the games I felt genuinely exhausted despite just sitting in my chair for a few hours, probably because I was thinking and calculating as hard as I could the whole time. Some high level players like Sam Shankland are known to frequently cite physical fitness as crucial to their ability to perform in high level tournaments, where they might play that type of intense games all day for multiple days in a row. (https://www.milibrary.org/tuesday-night-marathon https://www.milibrary.org/tuesday-night-marathon, though I'm not playing the currently running one. You typically play just one game every Tuesday night for 10+ weeks in a row)