Never leave your smart phone charging and plugged it while you are asleep, especially not under your sleeping pillow. There have been many cases where the phone battery got heated up during charging and burned the head of the victim person while they sleep. There is a high chance of the phone battery exploding and causing injury to you. I know people might say but it produces very small current and harmless voltage, not high enough to cause any burns. Look at the news and it will be prove enough to convince anyone about the dangers of such practice of leaving phones on charge while you fall asleep. If you need to charge your phone very urgently and also you are very sleepy, it is a good idea to ask someone else to keep a watch on your battery charged level and tell them to unplug it for you when charging is done.