This is someone that can easily answer in a flick of a finger. What I'm saying is the explanation scientifically. So, let's start with the basics.
First is the screen you're looking at. It's a LCD(Liquid Crystal Display) screen with implemented technologies like AMOLED. There are 2 types of touchscreens. There are resistive, there are capacitive. The difference is that the resistive needs more pressure so that the touch will be registered. Compared to the capacitive. Capacitive are what composes most of touchscreens today.
LCD emit RGB lighting. RGB stands for Red Green Blue which are the colors that makes up an image. Now, research shows that when we are directed with blue light ray to our eyes, our minds somehow wakes up.
And I've proven that because I have an app in my smartphone with a blue background. I was sleepy and when I opened that app, my sleepiness decreased. The effect lasts long enough that it makes you wanna use your device again because you're not drowsy.