Why does my brain stop working in the evening?

[Blackjack Davy]

I have quite a demanding job which probably answers the question, I need to be 100% on the ball from the minute I jump in the car in the morning to the minute I get home. No time to rest, no time to shut my brain down and go into auto pilot.

Well theres your answer, your brain goes into powered-down mode after you knock off work, neurons get fatigued too. Happens to a lot of us and it doesn't have to be brain work either getting physically tired has a similar effect.