Is it dangerous to keep my phone plugged in indefinitely? I just recently got a new phone, so my old one has been sitting in a drawer for the last month. However, a friend recently suggested the Sleep Cycle alarm to me, and I'd like to try it out, but I'm not willing to put my nice new phone under my bed every night and run the risk of it overheating or breaking.

However, my old phone, a ZTE Valet (Incredibly cheap, all-around low quality phone), I couldn't care less about. So my plan is to plug that in next to the wall, and use it exclusively for the alarm. I go to bed, I set the alarm, I wake up, I turn it off. It never leaves the wall.

However, I'm concerned about safety. It'll be in airplane mode and I'll disable other background apps that I can (probably just factory reset it before doing this), but is it dangerous to keep my phone constantly plugged into a wall on top of bedsheets? I know this might be bad for the batter (don't really care, I'm not doing anything else with the phone anyway), but I'm much more worried about any potential overheating problems that could burn me or start a fire. Should I be?

EDIT: I don't think this is a duplicate. I really couldn't care less if the phone is harmed by the constant charging, I just want to know if it's dangerous.
As you've said it is "incredibly cheap, all-around low quality phone" yes there are chances of overheating and all. I suggest you to charge it only at nights as you need it only for alarm.