Reading from a tablet or smartphone in the dark is okay for your eyes, as long as this is not for a long period of time. There is good lighting from these devices, with good contrast. There is, however, the blue light emitted from these devices. Blue light is a short wavelength light, with high energy that may cause damage to the structures of the eye if exposed for a long period of time. As well, studies have shown this blue light can disrupt melatonin production which is required for a healthy sleep cycle. Doctors of Optometry recommend limiting screen use during the last hour before bedtime.