Besides what you said of knowing what to do and the decisions part of the work there is one other thing, that at least for me, is(was) the culprit of most of my tiredness and irritability over the last year.

Changing contexts fast whilst taking decisions on these different contexts.

I was the only tech guy in this startup and I would handle basically everything on the tech side, working daily in three different languages and various "services". After a while as long as I had planned what I had to do for the day/week things were fine, but somedays I might be tackling this hard backend problem and I get asked to fix a frontend bug right now, I fix the bug, deploy it and go back to the backend stuff, couple hours later, I get a technical support call, again I must stop what I am doing to fix whatever needs to be fixed.

So, this thing of going back and forth between the areas in which I was responsible for would really take it's toll on my work, not necessarily in my productivity/hour, but in the amount of hours really worked, amount of sleep needed(which I found quite curious) and the chance of having some small anxiety crisis(which is something I kinda have to coupe with).