If you're on Android, I recommend Screen Filter[1] which will draw a semi transparent overlay on top of nearly everything (depending on the phone and/or OS version, it might not dim the status bar, bottom navigation buttons, lock screen, etc.). I use it a lot when reading at night, because the lowest brightness setting on my phone is also too bright for dark-adapted eyes.

[1] https://play.google.com/store/apps/details?id=com.haxor

Shades is also a good app on android for dimming the whole screen much darker than the default slider will go.

Wow, thanks a lot. Just tried it out and I'm loving it so far. I really appreciate the default reversion in case the slider is set too low, as well as the non-linear percentage slider allowing for fine-grained control at lower brightness values.

For iPhones you can set a full screen “zoom” with low light filter on. It’s under Settings -> General -> Accessibility -> Zoom. Set the filter to Low Light, then turn on zoom, then double tap with three fingers to zoom out all the way.

What phone are you using?