"What happens in the body when our eyes are exposed to blue light on electronic devices?

Chang: Recent studies have shown that short-wavelength [blue] light has a greater effect on phase shifting the circadian clock and on melatonin suppression. In 2014 my colleagues and I examined the effects of reading on a light-emitting device compared with reading a printed book. Participants who read on light-emitting devices took longer to fall asleep, had less REM sleep [the phase when we dream] and had higher alertness before bedtime [than those people who read printed books]. We also found that after an eight-hour sleep episode, those who read on the light-emitting device were sleepier and took longer to wake up. In the study all participants had to stop reading and turn off the lights at exactly 10 P.M., even if they did not feel sleepy. At home, I would expect people do not have the motivation to turn off their devices and go to bed, so they would stay up longer and experience even more circadian delay and shorter sleep times. The effects in the real world could actually be even greater."

"In the 1990s my team performed more than 700 experiments over seven years to measure how different wavelengths of light regulate acute melatonin production. Unexpectedly, we found that humans display a peak sensitivity to light in the blue wavelength region of the spectrum."

http://www.scientificamerican.com/article/q-a-why-is-blue-light-before-bedtime-bad-for-sleep/