There is some setting in wallpaper, or display called Daydream, that plays colours etc when the phone is charging, other than, it's possible that whatever charger you are using is intermittently dropping the charge power. I have an adapter in my wall plug that has three outlets and 2 USB on the side, if I have my phone plugged into one of the USB and the wife plugs her iPad into the other, they will both alternate charge and not charging, weird, just not enough juice for both, but that causes my screen to go dark, then light up again when it thinks it has been unplugged. check to make sure you have no power fluctuation. I will get notifications when my screen is asleep, by sound, but nothing visual