Everything matters in pool. It's a mental game as well as physical. I firmly believe in the benefits of eating right. I get kind of nervous, so I don't drink coffee before the tournament, because I know it makes it worse. Also I try not to spike my blood sugar.
I recently played a tournament and I was ill with the flu, it was just setting in at the start of the tournament. I was sweating and stumbling around and my brain was completely fried. I still managed to win a few matches, but I couldn't think clearly and I only managed to run routine racks. After years of playing the "autopilot" is not too bad, but when it gets rough, it's not up to the task.
I think the strangest part of being sick like that is the fact that it's almost impossible to get into the zone while playing. It's like watching yourself on tv, rather than being there 100% like you would normally. No matter how hard I try, I just can't get in the right mindset.