You know you're tired when ...

You drop 60 points in a half hour losing streak, with most games under 25 moves. Chess is addictive, but not truly relaxing.

you blunder each of your game. 

you fall asleep.

... you're too tired to know how tired you are

That was me last night and the other week.

Other week I dropped about 100 ELO, from 1250 at blitz to 1150.  Then I worked my way up to 1280 by winning a lot yesterday.  When it got later, it deteriorated.  Now I'm barely above 1200 again.  I need some consistency.  

@reflectivist yes, consistency is a bit tricky. I think you have to discover a rhythm. Of course, this from a guy who yo-yos at around 1250. Simply, don't play when you're out of sorts, defensive, or persisting.

You try to fork your own pieces

you take short naps whilst playing blitz game

you try to checkmate the queen

When you claim en passant is cheating even though you used it earlier in that same game

ZzzzzZzzzzzZzzzzz.... oh, sorry, what was the question?

you rest your head on the board, knocking off the pieces.

There is time to improve the rating. Usually 8-10 p.m. sometimes 8-11 p.m.

Chess is around the world. There is time difference. So, when you are tired, others maybe fresh.

You have this position as white, it's your turn to play and you still find a way to lose.

hahahahaha   I 've done that!!!

Good Job!

When you take your hand off the mouse and it is still the same shape

2 Minutes later your hand is still the same shape of the mouse.

Get yourself a footbath, put some music on and a plateful of oreos and some strawberry milkshake, get your friends on skype and play bullet chess like there is no tomorrow and the odd blitz game ! I limit myself to a certain number of games and how much points I can afford to lose in one day, about 30 points max!  

Your opponent plays Ke1-b4+!? You write down his move and play Kf8-e8. (looks like your both tired!) 

lol