Is it okay to charge the iPhone overnight? (8 hours)

Yes. It is actually recommended to do so. The iPhone will learn your patterns and manage the charging overnight so it's ready in the morning. It will charge over night in such a way as to preserve battery life.

It cannot be overcharged and no harm will come to it by leaving it plugged in overnight.

I do that since 2008 with all my iPhones. I've always charged them overnight, everynight.

No harm and no overcharging, since the device stops charging automatically when the battery level reaches 100%.

Regards

Giulio