I would imagine 7 hours would be tiring to almost anyone, it certainly is for me, especially with only a 5 minute break on the hour from a tournament ... that being said ... you adjust, because it is something you love to do ... and mostly I play cash games online, so it is easy to sit out for whatever reason, and go back when I am ready ....
Probably if you just listen to music or surf the net or anything else but poker, while you play, it is fine but if you only concentrate on the game... watching others play ... making sure you are on top of your
odds .. ranging your opponents etc .. I don't see how it couldn't be somewhat mentally stressful and tiring ... you just have to make the most of your off table time and have some balance then you can deal with it more positively .. at least for me : )