Rechargeable batteries used to suffer from memory effects – whereby if they were not fully discharged during each usage cycle, their capacity would diminish more than would be otherwise expected. But with modern battery technology, this has been eliminated.
Modern battery packs should be good for at least 1,000 charge/discharge cycles, so with maydup using her electronic device free from the mains charger connection each day – the battery pack should be good for around three years.
With regards charging overnight (unattended) this should not cause any problems – but to be on the safe side, make sure that both the mains charger and item being charged are away from soft furnishings and alike, that might add fuel a fire (should the worst occur).