I get asked about this almost every day at work. Will it make my eyes worse? Is it why my eyes feel tired? Should I turn the lights up? No. Probably. Yes.
Our eyes have to work to focus every time we are reading or looking at things up close. It's called accomodation. Little muscles inside the eye cause the little lens inside the eye to change shape so near objects come into focus.
It doesn't take a genius to figure out that things are harder to see in the dark. So, in the dark, the stimulus for accomodation is decreased and our eyes lose focus.
Our eyes will continually lose focus and then re-focus causing them to fatigue and strain.
There is a little myth to it though. Your eyes won't "get worse" from reading in the dark. By this I mean there won't be any change in prescription or need for stronger glasses.