JustFor the love of god, buy a lamp! A little table lamp or perhaps a standard lamp, depending on where you're reading. Or else if there's some special need for darkness, look into buying a torch, many are rechargable.
You can even get, quite inexpensively, the sort of lamp cave explorers use. Straps to your forehead. The better ones are rechargable, the cheaper ones aren't. Although you could get rechargable batteries and a charger to counter that, since this is something you're going to use a lot you don't want to shell out for endless batteries.
A phone is not designed to make a good lamp Using it's camera flash LED gets hot and eats battery massively. They're useful in emergencies cos you've always got your phone on you, but it's not an everyday sort of thing.