There is something out there called the law of diminishing returns. While this particular law generally refers to resource allocation and productivity, the words can apply to somebody sitting at a poker table. Winning is not a constant state, and for a lot of people their best poker is played during the first few hours of a session. Just like the least amount of work gets done on Friday at businesses across the planet, or near 5:00 pm on any standard work day, the longer a player stays in a game, the more the quality of their play will degrade.
In the game of poker, players should ask themselves, who the relevant players are, what their playing styles are, how many chips they have, how many chips you have, what your pot odds are, what kind of hand you have, what kind of hand you think your opponents have, and much more. But some poker players leave out a few important elements, such as their emotional state and their goals.