Overnight charging will cause no significant harm to battery life, and no problems. There are a lot of myths surrounding problems with overnight charging, but as long as you use the correct charger and follow manufacturer instructions, there should be no problems.
Firstly, your phone and charger, especially since technology has advanced, communicate with one another. When your battery reaches 100%, it sends a signal to the charger to start a "trickle charge" cycle. This basically means that the phone and charger shut off charge, and begin to send bursts of information to one another as to the status of your phone's battery. If the battery level has dropped slightly, the charge will continue, but at a trickle pace, and only for a tiny length of time, then it shuts off again. This makes impact to the battery life pretty much negligible, and also makes it impossible for your battery to overload.
Secondly, like any battery, your phone's battery begins to lose the capacity to hold its charge over a length of time. This is the same with any other battery in the world. However, in today's modern phones, this difference over the lifespan of the phone, is pretty much none existent.
Top and bottom, use your common sense:
Do not place the phone on anything soft, like a pillow or your bedsheets while charging, as the phone will heat up during this time.
Use official products, so they can communicate with one another in the correct way and gauge exactly what needs to be done. Third party products may not have correct timing on trickle charge, or may not communicate correctly at all, causing long term problems.
I have my phone overnight charging all the time and in all of this time, never have I once had a problem with overnight charging. I have always used official chargers, good quality cables, official wireless docks etc.
So, short answer, you should never have a problem with overnight charging.
Overnight charging myths debunked here:
https://uk.pcmag.com/android/92897/charging-your-phone-overnight-battery-myths-debunked
