There is no issue; 20% overnight would not be unusual. Any cell phone uses energy continuously, as long as it isn’t powered off. This means that if it is not connected to power for 8 hours the battery level will be lower at the end of that time. How much lower depends on what apps you have, how many notifications you have enabled (which will be delivered and processed by apps even if you force close apps-they will relaunch when a notification arrives), and, probably most important, how strong your cellular signal from the network is; your phone maintains a "conversation" with the network so your phone can be found when a call or text arrives, and low signal strength requires much more energy to do this update to the network. Thus, you should be charging your phone overnight so this nighttime usage is powered by the external source rather than the battery.