@fast,
I often do long days of classes. Afternoons are often 5.5 hours with only a twenty minute break. That's intense. There's no time to relax, it's full on stuff, so if I do start getting a headache, from the noise, the mental activity, the general concern and care of teaching teenagers, I generally pop a couple of paracetamols. Yeah, I know, it's probably bad for my health.