When I was suffering from this I tried to hack it and what I noticed was that when I got more blood to the brain (e.g. by being upside down), it improved (for a minute or two). So that led to the brain hypoperfusion theory and then I decided to ramp up the red blood cell count to give my brain the ability to do more with less, by taking liposomal iron, lactoferrin and liposomal sublingual B12. This led to a massive boost that I could work normally for 8 hours (but had to take rest afterwards).