Would you recommend playing cs when you're tired, stressed, or sleepy? Even if it's just for fun?
I've never felt that being tired or stressed has ever affected my play. I will never use those as an excuse for playing poorly. Chances are if someone is saying they are too tired to play or stressed, they are just being ignorant and using excuses to make themselves feel better. If you personally feel it is affecting your play, then your better off not playing at all, at least not with your team. Do not play with your team unless all 5 of you are giving 120%, otherwise it's just a waste.