How to increase stamina/mental endurance during slow chess otb tournament

I have noticed that after ca 3 hours of slow chess, i usually run out of stamina and start making game loosing mistakes. Thinking and focusing becomes much harder and i tend to rush the game just to get it over with it.

What do you do to stay focused, alert and fresh during a long otb game? Coffee doesn't work for me. I avoid sugar & quick carbs and eat more LCHF-inspired. I also make sure i get enough sleep and don't do any chess study day before to rest my head.

Beyond getting adequate sleep and exercise and doing LCHF, I can't think of anything else. Are your carbs low enough for you to be in ketosis? If so, that should reduce the brain fog and increase mental stamina and alertness during games.

Experience is the most important thing. As you become more accustomed to playing longer games and concentrating for longer period it will get easier for you.  As everyone has already said, sleep and exercise with both help a lot.  

drink redbull my guy

Standard counsel.  Nutritious diet, endurance exercise, lots of sleep, longer study sessions, positive mental attitude with a hunger for intellectual battle evidenced by body language and facial expressions.  

Eye of the Tiger, baby.

Well, Red Bull, 5-Hour Energy, or even just strong coffee are about all you can resort to once the game is starting, but decent eating and sleeping habits will do more for your stamina in the long run