Fast charge is immaterial to a question of leaving the phone plugged in over night. Nearly all of the "fastness" of fast charge happens in the first 60% of the charge cycle. Once you get above around 60% battery charge, it's basically a conventional charge profile.If you're going to charge your phone overnight, I would recommend turning fast charge off, since the extra heat of the faster charge will degrade the battery over time. Once it's charged, though, it just keeps the battery topped up and will not harm the battery.
Fast charge is immaterial to a question of leaving the phone plugged in over night. Nearly all of the "fastness" of fast charge happens in the first 60% of the charge cycle. Once you get above around 60% battery charge, it's basically a conventional charge profile.
Therefore, you can debate leaving it plugged in or not plugged in overnight, but fast charging has nothing to do with it.