I had the same problem - late afternoon headaches, tiredness. It was a colleague who took one look at how close I sat to the monitor to tell me to get my eyesight checked. Diagnosis: long-sighted in one eye, short-sighted in the other, which caused quite significant muscle strain while my eyes constantly fought eachother for focus, causing headaches and tiredness. Prescription goggles and the problems vanished.
Get your eyes checked, at best you get told you have 20/20 vision and you're mad, and to drink more coffee. At worst you're in line for glasses
"Get your eyes checked, at best you get told you have 20/20 vision and you're mad, and to drink more coffee. At worst you're in line for glasses"
I'd turn that best/worst around. Glasses are easy to get, cheap, and work deterministically and measurably. If you have 20/20 vision and still have headaches, you probably have bigger problems than the relatively minor nuisance of wearing reading glasses.
I didn't have late afternoon tiredness or headaches, but I noticed that sometimes after staring at a computer screen or TV for a long time my eyes would get entirely bloodshot. It would last about 2 days before it finally cleared. I would also get a pain, but it felt like it was my eyes rather than a headache.
It turns out my one eye has a slight astigmatism (I think that's the term) but it's just enough to need glasses for driving and TV watching. With my computer I've made adjustments so I'm not overly concentrating with my other eye. It had been about 5-7 years between my last eye exam because I thought I had 20/20 vision so sometime in between there I developed problems.
So even if you think you have 20/20 vision you still might want to go get an eye exam periodically.
Get your eyes checked, at best you get told you have 20/20 vision and you're mad, and to drink more coffee. At worst you're in line for glasses