The hour-four decline
Your A-game folds rivers correctly while stuck. Your C-game makes the call you would never make fresh. The dangerous truth is how much of a long session is spent in the latter.
Fatigue is a leak
Decision quality degrades with hours, not just with tilt. Late in a long session, marginal spots tip the wrong way, a hero call here, a loose 3-bet defence there. Spread across enough sessions, the hour-four decline can cost more than any single technical mistake.
Find it in your own data
Split your results by session length and time-in-seat. If your hourly sags in the back third of long sessions, you have measured your C-game. That is not a feeling to argue with, it is a pattern to act on.
A stop-clock, not just a stop-loss
Cap session length the way you cap losses. Quitting while still sharp protects the win rate the late hours quietly erode. The discipline is deciding the clock before you sit, not negotiating with yourself at hour five.
The leak hides inside winning sessions too
Fatigue is hard to spot because it does not only appear when you lose. A long winning session can include an hour of poor calls that the final result forgives. The scoreboard says good night, the habit says keep playing, and the leak survives. Splitting sessions by hour lets you see whether the late-stage decisions are actually adding value or merely riding the earlier profit.
Set the exit before the chair feels comfortable
A useful stop-clock is specific: maximum hours, review point, and the condition that overrides the room looking good. For example, check in at hour three, quit at hour five unless the game is exceptional and you still feel sharp, then write why you extended. The written reason matters because it gives the next review something to test. If every extension becomes a losing hour, the rule was too soft.