Is part of being a good chess player knowing when not to play ?

Hmm, first ask yourself how you feel.

Oh, that makes it easy then! Being really tired is almost always a good sign that you're really tired.

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Heh, but ok, people usually have a time of day or event against which they can mark their sharpness. Get enough sleep, wake up and eat a good meal, you'll probably have good energy... but you'll also probably be at work or school.

Some people get a burst of energy in the evening. Figure out your ideal time should be easy. It's the discipline to not play after it that will be hard I think.

By chance I've used a method often enough to test to see how rested I am. When I'm waking up I try to do some simple but long math problem in my head, then I check with a calculator. I have no talent for it, so I actually have to visualize and remember numbers as I go, so it's a good test. If I get it right, then I know I'm rested (i.e. have good mental energy, can focus, etc).

Yah if yah feel really tired yah probably are....

Seriously though

I'm sure yah can tell after a game if your performance was up to par

If not. Break tims