How to make your iPhone and iPad best for reading so it does not hurt your eyes for long reading.
There are some simple things you could do through Settings, though generally, as Dann says, it's better to read for long periods on a Kindle, Kobo or Nook etc.
My first suggestion is to change screen brightness. To do this: Settings > Wallpapers & Brightness > (Adjust brightness to suit you). Increasing text size might also help slightly: Settings > General > Text Size. Depending on what you're reading (blogs or ebooks etc.), your iPhone can automatically adjust its contrast; all you have to do is: Settings > General > Accessibility > Increase Contrast > On.
It's never helped me, but some people like to invert colours (ie. read white text on a black background). It does, I suppose, give your eyes something a little different to look at. To do this: Settings > General > Accessibility > Invert Colours.
Hope that helps!
Depending on the app that you're using to read, you might be able to enable a night mode or at least a toned-down look (like the Kindle app's sepia tone). You can also install f.lux if your device is jailbroken (https://justgetflux.com/ios.html). I use f.lux on my computer, and it's made a huge difference for my eyes at night. You could change the temperature of f.lux to make it easier throughout the day, too.
Other than that, I'd recommend off-loading as much of your reading to an e-ink device, like a Kindle, as possible.