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.