This is what I remember. Lets say you're playing a new video game. When you beat a new level, your brain releases a hormone that makes you feel good. Your brain releases this hormone naturally, but just like melatonin, the more you use it [or stare at a screen] the more dependent you are on this hormone. This explains why you are depressed after a lot of screen time. This is also a reason why it should be limited.
Hope this helps!
