I feel a mild headache coming on. I can guess what the causes are: lack of caffeine, possible minor dehydration, a little bit of tiredness - I slept last night, but not quite enough - eyestrain, and stress.
Time to stand up, stretch, drink some water, and get some coffee. The eyestrain might be less of a problem after I get my new glasses. The stress will be less of an issue when I get out of this place.