I feel the need to bring this to attention. Most often around here people mention certain unbalanced FR causing fatigue. What I'm noticing is that coherency of what I listen to reduces due to listening to headphones over very long periods which is likely causing lister fatigue and my sense becomes "tired" perhaps and reduces focus. This is not all, I notice that my hearing becomes unbalanced as result. One side becomes more sensitive over the other as akin to effects of equal loudness curve response. There are so much that I've experienced that are relavent to phychoacoustics such as headphones sounding different at different times I listen, which is likely related to my physical condition. Also, sometimes listening at night time, I have a duration of euphoria when things generally sound better. I also belive environmental factors has to do with it as well, temperature and humidity. When I listen in the morning after a restful night of sleep in my work desk, things generally sound better. I am human.