I wanted to make this post in case there’s anyone out there who hasn’t discovered this yet, like I hadn’t until recently. Sometimes when you turn the brightness down on computers, phones and etc it’s still too bright for someone with light sensitivity or migraines. I finally found this option on my iPad to make the light even lower and I only wish I had sooner. I don’t have an iPhone but I can imagine they would have this setting too.
Settings > General > Accessibility > Display Accomodations > Reduce White Point
You might also want to make sure Auto Brightness is turned off if your device turns the brightness up on its own.