I think if the challenges are just at the right level and the work is interesting to you, you can work almost as long as you want. But if the difficulty level i

I think if the challenges are just at the right level and the work is interesting to you, you can work almost as long as you want. But if the difficulty level is too high and you feel no sxcitement, it becomes stressful and and eventually you reach a point when you cannot go on. I had this with a project that was difficult and that I found not interesting at the same time. I could not go on. I would rather browse amazon or clean the livingroom all day than stting down to work on the project. I felt really miserable for this.