When the phone is fully charged the electronics in the battery tell your phone to stop sending electricity from the charger to the battery charging circuit. Power still comes in, it just doesn't work at recharging anything and goes back out to the wall socket (electricity travels in an unbroken loop to and from the power source). If your phone is still powered on the battery will discharge because it's being used. When it discharges to a certain point, charging starts again. This cycle repeats until you unplug it in the morning.