Blue light inhibits the production of melatoinin, which is what your body needs to produce so you can sleep.  Artificial lights are part of the problem, because biologically you are immune to the sun setting as you are always bathed in light, when you add the blue light from electronic screens, it makes it even more of a problem.

Intellectually KNOWING this is the problem and solving the problem, for me, are two completely different things, since i have so much trouble falling asleep, i made a no tumblr past 9 pm rule for myself, which is helping a great deal, i also try to stay of the laptop.

Upworthy Article on blue light and sleep