Nope never. i am experienced now, 16 - 18 hours in front of PC and now no problems both to eye and head
i dont feel any problem for sitting there for about 5-6 hours a day; one thing you can do to avoid any problem is to just have a walk of 10-15 steps after 40-50 mins, and look at some other objects than your screen after every 15-20 mins; would be a sort of rest for your eyes and body;
headaches are often from dehydration ... make sure you're drinking enough water regularly ... also get up every now and again and stretch!
Just sit in front of the PC for an hour and then just relax for 5 mins I do the same and am not having any problems.
Two other things : 1. Try blinking more - people tend to gaze when they look at monitors. 2. Try adjusting your monitor refresh rate - some people report certain refresh rates (like 60hz) give them head aches. gl