Prefrontal cortex
The front of the brain. Planning, holding back, keeping things in proportion. Works worse when you're short on sleep.
The part just behind your forehead. It plans, weighs options, holds a thought while you do something else, and stops you saying the thing you'll regret. Evidence: solid It's among the last parts of the brain to mature, and it's still changing well into your twenties. Evidence: decent
Short sleep, alcohol and long stress all make it work worse, which is why the 11pm argument and the 3am worry both feel so much bigger than they will at nine. Evidence: decent
Why it matters
When you can't think straight, the answer is rarely to think harder. It's to fix what's wearing it down: usually sleep. Evidence: decent