Then I've just went on google to find out why is this?
And here is why... It’s because your mobile phone charger doesn’t stop charging after your phone reaches 100% capacity. It keeps topping off the charge during the night. This is called a “trickle charge.” While you’re sleeping, and the phone is plugged in, it works to keep fully charged by compensating for the small amount of charge it loses by just being turned on. This is bad because the trickle charge causes your mobile phone to retain a higher ambient temperature than it should. This ultimately reduces the battery’s capacity.
I wondered why I couldn’t seem to keep my phone charged all day. It’s barely three-years-old, and I’m already having battery issues! Now I know why. I’ve been damaging the battery all this time by charging it at night while I sleep. I wish I had known this before!