Not sure about risks if more than the usual time needed to fully charge.
If a battery has 2000 milliampere of energy and the charger output of 1 ampere, usually it will take 3/2 of the time meaning your battery will be charged in 3 hours.
Personally I would not leave much more plugged unless you have charged it many time not fully and you try to recover the memory effect taken.
If you have only 1 battery or/and necessity to use heavy without other chance to charge it for a while, I will do, but as now usb connection is the same as the charging one, I think if you don't damage the battery or phone you may shorten the life of the charger or may sleep bad as sometime magnetic wave near you may keep you less able to enjoy a deep relax.
I use personally dock charger mainly those one piece with integrated plug that from Chinese online eBay and co. cost like your Sunday morning coffee and you can charge batteries using at the same time the phone without worrying of where you leave it charging.