[codingdave]

Different companies and teams have different cultures, so while I agree that the standard 8 hour workday isn't ideal for coding, and personally seek out teams that allow more flexibility... I also understand why some places do it.

That being said, if you are working yourself to exhaustion and burnout within those 8 hours, to the point of chronic pain... that is something wrong the way you are managing you work, your time, and your stressors. Many comments already in this thread are terrific in the details of this, and how you can adjust. But this also sounds like a medical issue. I'd get yourself to a doctor, and get some bloodwork done. You may have something as simple as vitamin deficiencies (probably due to how hard you are pushing yourself), so identifying them, adjusting your diet, and taking some supplements could help you. Go take care of yourself, and work will fall into place.