On Monday I did my first training session, and I spent 30-40 minutes and I possibly put in too much effort, because 2 days of rest later I'm still feeling significant fatigue - I don't work this week so I mostly rested in that time. I feel that I'll be tired for 1-3 more days. I strongly suspect that this is not typical.
This is a general thing which happens to me before HEG biofeedback - I would sometimes trigger a type of fatigue and headache which lasts for days. Generally after that I see an improvement. I could not find any scientific literature on this type of fatigue (or nonscientific for that matter). I can't trigger it by will and haven't been able to trigger it in the last several months.
So I was a bit surprised to see it trigger from biofeedback, and it made me wonder whether a similar thing happens to everyone else doing biofeedback, or whether my own type of mental thing overrode whatever the biofeedback was supposed to do.
Very interesting! I'll keep looking for an explanation for this.
Note that this sort of fatigue/headache has caused great improvements for me over the years. It took me a while to believe it, it had to happen multiple times until I thought that it's the most likely explanation. Basically a life-changing amount of improvement.
Another thing I've observed is that it makes me need extra sleep, and that I tend to spend a lot more in the REM phase - on one extreme occasion, I slept 12 hours, 4 of which were REM, but in general I also remember dreaming a lot more.
I have a few more observations, for example one time I was able to reliably predict that someone is able to trigger this headache/fatigue just by his behaviour online (he had trouble communicating some thoughts, relied too much on the exact meaning, was oblivious to broader meaning). And most people don't have it (which probably means that they do have it, but it's way weaker).