As a shift worker who works a pretty random schedule, I have the unfortunate opportunity to experience how fatigue and tiredness affect my abilities as a chess player. A look at my blitz rating here paints a pretty clear picture. When I'm on my days off, and when I've had a good rest the night before, I can often make my way up to around 2100 (I make no claim to ever being a very good blitz player, but 2100 is better than 1950). When I'm working a set of night shifts (as I am as I write this), my rating falls to about 2000.
I figure the first thing to go when you're tired, is your tenacity. When you're playing an opponent of similar strength, sometimes the only decisive factor is how hard a player fought. In chess, the will to win is incredibly important as it motivates you to 'dig deep' and calculate as well as possible (hopefully before it's too late). When I'm tired, I tend to get sloppy and I often let those 'slightly worse but still very complicated' middlegames that Sicilian players should be familiar with slip too easily. When I have more energy it shows on the board and it seems as though I'm able to turn "slightly worse" into "with counterplay".
I also find that fatigue causes my tactical and calculation abilities to falter slightly. I originally thought that this was caused by an increase in isolated, unpredictable, blunders, but after my recent poor performance on the tactics trainer I use, I now believe that this ability falters too during times of fatigue.
As I also mentioned, isolated blunders obviously increase when one is tired.
All said, I now believe that the single most important factor in terms of maximizing tournament results is getting a good nights rest. It's clear that a tired player is a weak player.
... time for bed.
Until next time,