Evidence is mounting that most e-readers and tablets, those with backlights, interfere with sleep. Several studies have found a correlation between exposure to blue wavelengths of light and subjects having trouble going to sleep. Data also point to blue light exposure altering the circadian rhythms that set the body’s clock. To be fair, this has also been voiced as a concern regarding high-Kelvin fluorescent lights, which also emit significant amounts of blue light. It has generally been cautioned that exposure to such wavelengths should be limited after sunset.
It should also be noted that some e-readers, such as some models of Kindle, do not have backlights. It is not thought that these create a problem, because the light they have been found to reflect is redder, (though one might presume this is at least somewhat dependent on the light source in use).
Ars Technica has a nice summary of the latest data: http://arstechnica.com/science/2014/12/e-readers-and-tablets-really-do-seem-to-alter-your-sleep-schedule/