Why does my phone die so fast when its always on lock!!?

So I would charge my phone then put it on lock for the night cause you arent supposed to charge it over night right? And id wake up and it would be dead! so Id charge it again and I dont really use it alot cause im always on my laptop so itll be lying around on lock(with the screen shut off) and Id get it when I want to use it and it would be dead again like 6 hours later!! Why? I thought phoneswere suppose to last a long time on lock?