It's hard to say how quickly overnight charging actually accelerates the aging process in the lithium-ion batteries inside our smartphones. Charging overnight may not produce any noticeable effects if you buy a new phone every year, or maybe even every two years.
But if you typically only buy a new phone because your old one is dying too quickly — and your phone isn't that old — you might want to re-evaluate how you've been charging your phones. An overnight charge might not be considered such a long period of time, but the damage could build up the longer you keep your smartphone. If you're an overnight charger and plan to keep your phone for several years, it might be time to ditch that behavior.
But remember, all lithium-ion batteries die over time. You can be the most cautious phone charger in the world and your phone's battery life will still degrade over time, albeit more slowly.