It was 3:47pm on a Thursday and I could feel my jaw was clenched. I'd been at the screens since before the open, made three decent calls, managed two setups that went nowhere, and then — right there in the final twenty minutes — I fired off a fourth trade that had absolutely no business existing. No signal. No edge. Just a vague, itchy feeling that I "needed one more."
That's decision fatigue. Not tiredness exactly — something uglier. Your prefrontal cortex has been making disciplined calls all day, and by late session it's essentially handing the keys to the emotional brain and saying "you handle it, mate." Mark Douglas wrote about this psychological state in Trading in the Zone — the idea that your mental framework erodes under sustained cognitive load, and suddenly the rules you swore by at 9am feel optional at 3pm.
The cognitive bias operating here is called ego depletion — the theory that willpower is a finite resource that drains with use. Every trade decision, every position review, every "do I hold or cut" moment costs something. I kept a session log for six weeks once and the data was uncomfortable: my win rate in the first two hours was nearly double what it was after lunch. My process hadn't changed. My brain had.
The fix isn't complicated but it does require admitting you're not a machine. Many systematic traders pre-commit to a maximum trade count per session — some set a hard stop after three setups regardless of the clock. Others treat the last forty-five minutes of any session as a no-fly zone. Douglas's central argument was that consistency comes from a stable mental framework, not intelligence. For further grounding, the concept of decision fatigue on Wikipedia is worth reading alongside what Investopedia documents about behavioural finance and emotional trading errors. The mechanics of ego depletion research on Wikipedia explain why even rigid rule-followers crack late in the day.
I closed that Thursday trade for a loss, obviously. Added it to the log, poured a beer, and wrote one rule in my journal: "If you're negotiating with your own process, the session is already over."
Your trading rules aren't suggestions — and they get loudest when your tired brain is whispering that this one's different.
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