It is definitely mentally exhausting if you are a perfectionist. I day trade equities, and I try to time all my entries and exits down to the second. I am glued to my monitor for the entire trading session, monitoring many stocks, all sectors, important commodities, etc. Watching 6 level 2 boxes at once.
Just like someone else said, a beer or a glass of wine is required after a day's work!
That being said, come on, this is the greatest job ever.
Every day is exciting, you could make more money TOMORROW than you ever have in your life.