It happens to me to. I play awfully when I'm tired. Don't blame yourself
Playing tired

Sounds like me this past week.
I'm realizing chess is not like playing solitaire or a video game on the internet. Yes we can play for pleasure, but chess is not a game to just sit and play lazily. If we do, we'll suffer the consequences. Like you, most of my games at night, especially late at night will be under 15 min games because I know I am not gonna take my time to evaluate as much etc...like we are suppose to do.
Now I also see that my over 15 min games are played badly as well.
So here's my take on it... 1) Either I am having just a bad day and it will now go up or 2) By playing badly at night or any game that I decide not to take to seriously, I become lazy and I am taking the same thinking patterns into my so called good games. I'm also playing too fast for the time control allowed.
So I'm thinking consistency is the key...No matter what game is played, we need to give it our all so that when we play any type of timed game, we will continue to use the proper thinking patterns required to win at chess.
Ok ya 1) I've lost a Queen because the doorbell rang,
2) I made a stupid move and lost a game when I was winning because my baby girl was crying and needed changing :)
3) I made a stupid move because my wife had something to tell me.....
...Why are we playing late at night again ?

This whole month I haven't had much sleep. This weekend was real bad though. I had 5 hours the whole weekend. My cousin just bought a new house and I was helping him get some things squared away.
I love to play chess and was having withdrawls from it by the time I got home so I really wanted to play before going to bed. That was probably a bad idea. In retrospect, I wasn't really even thinking. Nor did I play with a plan. I just moved pieces to squares that were familiar while I listened to my itunes. I should have just went to bed. Now I feel like I'm in a slump.

Sounds like me this past week.
I'm realizing chess is not like playing solitaire or a video game on the internet. Yes we can play for pleasure, but chess is not a game to just sit and play lazily. If we do, we'll suffer the consequences. Like you, most of my games at night, especially late at night will be under 15 min games because I know I am not gonna take my time to evaluate as much etc...like we are suppose to do.
Now I also see that my over 15 min games are played badly as well.
So here's my take on it... 1) Either I am having just a bad day and it will now go up or 2) By playing badly at night or any game that I decide not to take to seriously, I become lazy and I am taking the same thinking patterns into my so called good games. I'm also playing too fast for the time control allowed.
So I'm thinking consistency is the key...No matter what game is played, we need to give it our all so that when we play any type of timed game, we will continue to use the proper thinking patterns required to win at chess.
Ok ya 1) I've lost a Queen because the doorbell rang,
2) I made a stupid move and lost a game when I was winning because my baby girl was crying and needed changing :)
3) I made a stupid move because my wife had something to tell me.....
...Why are we playing late at night again ?
Good point. You hit the nail on the head. I have a tendancy to play lazy. Chess is not meant to be a relaxing leisure activity. It requires thought and exertion. I many times give it neither, so I reap what I sow. I think my laziness has infected all parts of my game. I need discipline. I should only play when I am willing to give it significant effort.
Thanks. That's really my problem. I'm a lazy freakin slob who wants a cheap victory. I should be horse whipped until I straighten up.

I lose even more rating in Tactics trainer then I am tired about 200 rating points.
but it is always easy to get them back like cutting in hot butter when I am not since the assignments 200 ratings points lower are easy if I am not tired.
Sleep can have a huge impact on you're performance and results if you play chess tired you'll lose without the proper amount of sleep and rest you're brain and body is not going to cooperate at all there isn't a point playing.
I once learned that the hard way when I played in a school tournament on a sunday after an exhaustive week of stressful school exams many years ago lost every game :(

That's one reason I don't play much live chess. I don't have a lot of time to play a game during the day without interruption, and in the evening I'm usually just too tired to focus on a game. I don't even like making moves in my online chess in the evening unless I'm quite certain what moves I want to make. And even then I'm not above screwing it up.
Lack of sleep tremendously lowers my playing skill. I normally catch blunders and know how to escape attacks, but a few nights ago I was playing at 4am and made 6 blunders in a row, including not catching my opponent's blunders. I finally lost that game. After analyzing the game, I thought I was losing my mind. Then I realized if I'm losing my mind, how am I able to see the blunders now? So obviously just sleep deprivation. It's nice to see I'm not alone and that sleep is the reason.

It seems the impact on blitz is even bigger than on long games ... after all, you don't suddenly lose your chess skill if you are tired (but still are a lot more prone to blunders!), but most of all your thinking is much slower. Which does not have to mean defeat in a long game, but much more so in a blitz game.

Chess is an excellent measure of our tiredness, or otherwise.
WELL SAID and so true !! Because serious chess demands Total concentration, personal problems or lack of sleep destroys any hope of really playing at your best.

I play blitz games on my phone in the morning, on the bus. I typically lose those games, however I don't tend to play blitz games as I find them relatively useless in preparing for a G/30 tourney coming up. I lose most of my blitz games in the morning, when I'm tired.
Alternatively, I also play blitz games on the bus on my way home, when I'm more attentive and awake. I win most of these. I beat a player who was ranked 400 points above me.
Obviously this is all speculative, but it's a pattern that I have noticed.
Does fatigue really have a negative impact on chess play or am I just a patzer?
It does when you're very tired you can make very embarrassing mistakes, miss mates,blunder at the last moment,concentration will be way off the mark than it normally is.
When the brain and the body say no forget it!

if you're tired to the point where you make mistakes game after game then you should just play unrated. but if you're always playing while you're tired (like me)eventually you will get better at it, just takes time
I play far better late morning - when I've woken up and haven't hit the post lunch dip. Any other time and I'd be better off reading a novel. Also I'm addicted, so get worn out through too much play. This morning I woke up "woolly headed" after only five hours sleep, but my addiction drove me to play my drug of choice (15:10). I lost two games in the middle/end game just through not thinking - the last one I couldn't be bothered watching my K side while defending against a Q side pawn advance and missed a one move checkmate (!) My moniker tag says my profession is "Comedy chess"... this meant to be a joke but it seems to be spot on...
P.S. Climbing the curtains doesn't wake you up :)
So, I had a very busy weekend out of state with very little sleep. I drove 4 hours and got home late last night and sat down at the PC to listen to music and unwind with a little chess before going to bed. I decided to play blitz because it isn't serious and I just wanted to zone out. I normally win most of the games I play. Last night, however, I lost 10 in a row! I made stupid blunders and missed all kinds of things. My blitz rating dropped by 80 points.
Do you think it was just because I was tired or am I just really terrible at chess. I know I'm not a good chess player, but I didn't think I was that bad. I went to bed wondering why I stunk so bad. Did I just forget how to play?
I've also lost at online chess a lot recently. Since 8/14/11 I am a dismal 9-13-1. My rating has dropped 50 points there as well.
Does fatigue really have a negative impact on chess play or am I just a patzer?