As mentioned earlier in here: Getting Serious With Poker? according to research playing tired is almost as bad as playing drunk.
Now another research shows same thing again. In The Journal of Neuroscience was published results of research in where gamblers were tested both in normal condition and when tired.
"Sleep Deprivation Biases the Neural Mechanisms Underlying Economic Preferences" research by Vinod Venkatram, Scott A. Huettel, Lisa Y. M. Chuah, John W. Payne and Michael W. L. Chee says:
"Whereas well rested subjects tended to minimize losses, the same subjects tended to maximize gains after a night of sleep deprivation."
"A single night of sleep deprivation (SD) evoked a strategy shift during risky decision making such that healthy human volunteers moved from defending against losses to seeking increased gains."
To put it in short, when player is tired he will take huge risks seeking fast winnings. Even those players who normally wouldn't take risks.
The links to published research results in The Journal of Neuroscience:
Sleep-Deprived Subjects Alter Gambling Strategy to Maximize Wins
Sleep Deprivation Biases the Neural Mechanisms Underlying Economic Preferences
Another research proves again that you shouldn't play poker when you are tired
