I leave my iPhone plugged in, charging all night. When it reaches full charge, perhaps in the middle of the night and it is left charging, is it harmful?
Welcome to iMore! Just as @SwitchBeach stated, charging overnight won't harm your device. I've been doing it for years without issue. That being said, I wouldn't leave it plugged in for days at a time.
I charge mine during wake hours. Often times while using it. Sometimes leaving it plugged in a while after it reaches 100%. No issues from that as far as I can tell. I like having a full charge at bedtime.
There's circuitry that prevents over charging. The problem happens when you use knock off chargers that don't have the circuitry, or that have the incorrect volts and amps. (It's more so the bricks that are the problem, not so much the cords although you should always use apple cords OR apple certified cords)