Concentration

I was just thinking how focused one has to be in order to be able to act on stage. For me simply "seeing" or "hearing" something requires a lot of concentration and I am actually thankful that the class is in the morning because otherwise I don't know whether I would be able to be that attentive to everything. It's easy taking acting as a class when one can actually pick the time that best suits him/her but I wonder what happens when that one has to actually go on stage and it is not the best time of the day for him or her. A lot of jobs become a habit. Doing the same thing over and over again does not really require any specific effort or concentration after a certain period of time. Acting seems to be different every single time. I wonder whether one can actually get used to acting...