I really have to have the right mindset or I can find myself concentrating on something else and just auto-piloting the tourneys. Cash games are much easier to stay focused for me.
I also seem to have cycles, sometimes I will go for 2 months ready to play 12-16 hour days, and suddenly I seem to hit a wall. I will then stop playing, except for maybe a MTT or 2 on a Saturday or Sunday. Then after a break the fire comes back and I am ready to go all out again. For me it is not about being physically tired, more of a tired brain I think.