This post from the Business Insider is one of the many things that we already know but we are virtually incapable to inculcate in our daily lifestyle due to addiction to our "screens". Dr. Dan Siegel, a clinical professor of psychiatry from the UCLA School of Medicine explains why and how the screen time affects our sleep and more importantly our mental health in the short term. Multiple researches are also ongoing about the long term effects of prolonged exposure to the screens before bedtime.
Speaking logically, our bodies are not meant to be awake for long after the sun sets due to lack of the light (aka photons). And since we all still maintain the need to get up early in the morning for work, we are really not allowing our body and mind to rest the way it needs to, in order to function at a 100% the next day. So although you might be sleeping for 7 or even 8 hours everyday, the quality of sleep is really not of the optimum levels.
For additional reading on other health problems caused by the modern technology, you may want to read this post from The Huffington Post.
Simple advise - stay away from the devices atleast an hour before you plan on sleeping.