I would definitely look at the ergonomics of your work environment. Do you work on a laptop all day? I don't know how people can do that, it is terrible for you

I would definitely look at the ergonomics of your work environment. Do you work on a laptop all day? I don't know how people can do that, it is terrible for your neck and back. Something like this could be causing (or contributing) to your fatigue as much or more than the intense thinking.