According to the New York Times, there's no cut and dry answer. There is no evidence that reading on an e-book will harm your eyes. In fact, the lighting is very important. Devices with e-ink technology are best for use in sunlight or other well-lit environments. If you don’t have proper lighting, these device can increase the chance of eyestrain. But e-readers like iPad, which have a backlit screen similar to a computer monitor, are easily viewed in a dark room, will become reflective in natural light or bright sunlight, resulting in glare that can cause eye fatigue. Actually, long time reading, no matter on a e-reader or traditional paper books, will cause sore, tired eyes. So take a break every 20 minutes to protect your eyes, and don't get too close to the screen.