The claim that reading in the dark is bad for you, and leads to permanent eye damage and vision loss, is a claim that has existed for many years. There is not, however, scientific evidence to support this claim.
Some circumstantial evidence has been proposed to support this idea. Studies have shown a correlation between greater levels of education, requiring more reading throughout the life span, and worsening of eyesight in individuals. Other studies in very young developing animals have suggested the possibility that strain on the developing eye, very early in life, can effect vision later.
Despite this evidence, the vast majority of ophthalmologists are in agreement that while reading in the dark may cause strain on your eyes that can lead to short term discomfort, or even a headache, no long-term damage is actually done, especially in the case of the mature and developed eye. Hope this helps!