http://www.nytimes.com/2011/08/21/magazine/do-you-suffer-from-decision-fatigue.html?_r=3&pagewanted=all

The act of making decisions saps one's mental energy. A game of Go is a long series of decisions.

I noticed this in my tournament game yesterday. By the late midgame of the third round, I was pretty haphazard, and lost by 6.5 or 7.5. I was blaming it on caffeine (I'd been sipping cola drinks, and I've heard there's research showing caffeine makes you more alert but trashes your ability to solve logic puzzles), but maybe it's just the nature of the game.

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Interesting! Perhaps one of the things separating stronger players from weaker is simply the physical ability to continue making decisions longer. I wonder if that is also part of the real explanation for the decline in performance among pros as they get older. We all have fuzzy ideas about what "aging" is, but maybe the competitive effects are quite specific.

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Dave Sigaty
"Short-lived are both the praiser and the praised, and rememberer and the remembered..."
- Marcus Aurelius; Meditations, VIII 21

The round immediately after lunch is always the one I play the worst at.

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That which can be destroyed by the truth should be.
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My (sadly neglected, but not forgotten) project: http://dailyjoseki.com

I'm glad I'm not the only one!

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Dave Sigaty
"Short-lived are both the praiser and the praised, and rememberer and the remembered..."
- Marcus Aurelius; Meditations, VIII 21

When I was getting started with go, I would sometimes find myself utterly exhausted after or even during a game. Partly this was due to the stress of competition, but also it was just a matter of not having the stamina for such a prolonged effort. Now that I've developed a bit of routine, the games are less stressful, but stamina is certainly still an issue.

I often play fairly slowly in the opening, and by the time the middle game has started my energy is fairly depleted, and since there really aren't any situations where you can play without thinking, a moment's lapse can do you in. Losses of this type, where you fail to make a correct move not because you don't know it, but because your concentration has fizzed, are particularly painful.

One applicable idea from Kaz for his adult students is to specialize in one fuseki, so that when you have the opportunity to play it, you already know the ins and outs and can save your energy for when the fighting begins.

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Patience, grasshopper.

“When action grows unprofitable, gather information; when information grows unprofitable, sleep.”
― Ursula K. Le Guin, The Left Hand of Darkness

(I guess sleeping in the middle of a game may not always be possible!)

This is why one should play a lot of miai-generating moves.

I think it's the other way around. Caffeine helps you solve logic puzzles but it is a tricky balance, too much caffeine makes you jittery and impatient.
Nerves sap a lot of strength, too, and being in a tournament seems to go along with nervousness or anxiety which work against your ability to concentrate for long periods of time. As for the round after lunch, after you eat your body produces chemicals that are conducive to resting and the digestive process sends blood to the stomach and intestines, making less available to the brain. It is well known that music performers and actors tend to avoid eating right before a performance. Certainly you shouldn't eat heavy, greasy foods like pizza for lunch at tournaments

I've never read about it inhibiting your performance on logic puzzles and couldn't find that in quick google searches or on wiki, which seems pretty high quality for the relevant articles.

I do seem to recall that caffeine had a very strong positive effect on your ability to make estimates, but I can't find that either.

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Occupy Babel!

If seen professionals drink coffee during their games, and I remember many chess master are almost addicted to it ^^

This is all very interesting. I've had the vague sense that my yose has been getting worse in the last few months, whereas before that I kinda expected to start even games behind and catch up in yose. But it would make sense if putting out more effort in the opening and middle-game was leaving me too exhausted to think properly in the end-game.

In blitz games its so fast it wont matter as much. In slow games though, I usually take a break when I start to feel that drop off. Get a drink, go to the bathroom, maybe walk or exercise.

I think this also is one of the factors that separates professionals who have a quick rise to the top and rapidly burnout vs. those who have the ability to play and compete at the top level for decades.