Decision fatigue is the well-documented decline in the quality of decisions after a long session of making them. For a trader making dozens of rapid judgment calls, it's not a minor factor — it's a direct explanation for why late-session trades tend to be sloppier, more impulsive, and less profitable than morning ones.
How it shows up in trading
As your mental reserves deplete, you take the path of least resistance: you stop doing the full analysis, skip the checklist, act on gut instead of read, and become more prone to the one-more-trade and revenge impulses because resisting them also takes willpower you've spent. The afternoon “I don't know why I took that” trade is often just a tired brain defaulting to impulse.
Why the market compounds it
Fatigue peaks in the afternoon — which often overlaps the low-quality midday and early-afternoon tape. So your worst decision-making meets some of the worst conditions: tired trader, choppy market, marginal setups. It's a recipe for donating back the morning's gains, and it's structural, not a character flaw.
Discipline is a battery, not a trait. By 2pm it's low — so the fix isn't “try harder,” it's trading less when the battery's spent.
Managing it
Front-load your trading into your sharpest hours (often the morning), set a hard stop time or trade budget for the session, take real breaks, and lean harder on pre-made rules when tired (they don't fatigue). This is also where automation earns its keep — a rule-based system manages an open position with the same quality at 3pm as at 9:30, no matter how drained you are. Trade your best hours; let structure cover the rest.