I think it's optimal to take your phone off the charger immediately when it's fully charged but leaving it overnight will only lead to slightly accelerated degradation over time.
Look at it like this...these batteries have a finite life of X amount of charge cycles and, for example, charging it from 50% to 100% twice counts as one cycle.
From what I understand, once your battery reaches 100%, it'll keep cycling down and up between 95-100% to keep from overcharging. So if your phone is charging in 5% increments multiple times overnight, that's eating into the X amount of charge cycles every so slightly. Not sure if those 5% increments really make a difference in the long run but theoretically, I believe taking your phone off the charger immediately is the best practice for long term battery health.
Personally, I charge my phone at night and time it so that it's fully charged right before I go to bed. I usually get around 6 hours of sleep so my phone's at around 94-95% when I wake up which is more than enough to get me through the day.