Naturally, I get mentally tired, in particular during bad runs in cash games or rush tables.
Kinda weird but on really good runs I get more and more energy with every win and I can play for hours upon hours and then temporarily die in bed for 10-12 hours.
But if everything goes wrong in that session after 4-5 hours I feel like I've been run over by a train and probably need a couple of days of sun, cool activities, to get back in shape..
However, I do not get tired during tournaments. I can play quite a few at the same time and I really enjoy it. If I didn't have to eat, drink or sleep, I would play non-stop. But again, I play tight and take advantage of everything I can during the game. It may sound weird but tournaments are easy, or the decisions are easy. In cash games, it's not always easy and can get in tough spots quite often and the decisions can be frustrating at times.
There are days when my eyes start hurting and I have to stop. But what I can say is that it's the least stressful job I have ever had.
Hope it makes sense.