These are the headaches that occur when you lose part of your vision, seeing almost a fuzzy blindspot in your field of view right? I used to get a lot of this when I was a little bit younger. Nowadays, it's more rare, though I did get one a few weeks ago now that you mention it. Coincidentally, it was a day or some days after I spent doing an unusual act of heavy drinking.
In any case, I think getting enough rest, keeping hydrated, minimizing stress, eating properly, and getting some regular exercise helps.
No, the only obvious visual disturbance is an inability to get clear flashes, and usually it's caused enough strain to reset my progress. I think I had them start again a few times today. If so, it's the best I've done because I just closed my eyes for about 30 seconds, focused on the swinging period and breathing and was fine afterward. It's just strange that it happens so often. I must be doing something to generate them, but I'm not quite sure what.