For example, forcing myself to continue working at my computer when I don't feel like it will lead to eye strain, back pain and anxiety. I will often wake up with a headache after such marathons. I refer to this as burnout. Burnout is very real.
In my early years of programming I could work non-stop because I was doing something new almost every day. And for someone that likes new experiences, it was exciting. But after years of experience, programming in-and-of itself feels more and more like labour.
Don't get me wrong. I still love programming, in general. I've just become more focused on the end-results of programming--of what it makes possible, what it does for people. The act of programming itself, while being something that will always be a significant part of what defines "what I do", is no longer something that can sustain my interest full time.