I get these every so often. Usually, it is a combination of dehydration, tiredness or stress, and concentrating on a computer screen or something else that means focussing my eyes for a long time.
They start with a tiny out-of-focus patch in the centre of my sight, which gets bigger and more zigzaggy round the edges for twenty minutes or half an hour until it covers my view. There has no headache.
Once I recognise it (and stop blaming dust on my glasses) I stop what I have been doing and get a large glass of water and a couple of paracetamol or ibuprofen. A paracetamol and codeine tablet is even better. After that I have a hot cup of tea or coffee, which adds liquids and rehydrates me, and the zigzags continue to expand outwards and then to fade.
Another half-hour generally sees them gone, and I don't usually get the headache. If I take action quickly, I don't need the darkened room, but I do take it easy and quietly for the rest of the day.
The first time it happened to me was the worst. It was a couple of days after my first child was born. I was in my hospital bed (they kept you in longer in those days!) talking to my husband, and was getting my words mixed up. Then my side of my lip went numb, and one hand.
Told a passing nurse, who took my blood pressure and fetched a doctor, in case it was a stroke, but that was ruled out. My curtains were drawn and I was tucked up in bed for a nap. While lying there, I got the zigzags in my vision - and instantly stopped worrying about strokes. My mother was prone to migraines and always got the zigzags when one was starting.
The GP prescribed some tablets to keep in readiness, but I had no more until the exact same timings after the births of my other children, plus one after a miscarriage. Stress, tiredness, and dehydration again, and hormone disturbances contributing. After that, I was almost free of them until after the menopause.
At least when you know what it is you don't have to worry about what it might be!