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).
This is fairly normal as far as I'm aware, though it's definitely easy to overdo it the first few times till you get used to it. Keep it to a few minutes at a time for a while. 3 days of fatigue is pretty long, usually it goes away after a good night's rest if it's noticable, but you can think of it in one way like using a muscle you haven't moved basically ever. I am not sure if it would help to do a few minutes of it again now or wait a few days, that's up to you, but definitely slow down with it. Lots of people can do it several times a day no problem after the first few go's with it but there are definitely some known risks mostly to do with being careful with your vascular health. I know there are a lot of odd sort of psychological things that can happen when you start pumping the gas on your brain, too, possibly due to weaknesses or conditionong in different areas being hit on and triggered. I have not entirely understood that yet but it's a real phenomenon just like how different functions in our brain are physically mapped to different areas. It is generally not a fun experience but is really common, I'm not sure I'd call it dangerous as I have yet to hear of any serious side effects that were seriously attributable to using it. Drink lots of water too.