I never charge my phone overnight. Usually it is fully charged at around 11 PM.
I always have it turned on 24/7 due to my profession.
What I've noticed since updating to 8.4 it that, even with no activity at all, I wake up at 6 AM with a 75% charged, so don't know what happened in the night that caused a 25% drain.
Before 8.4, I was able to wake up with 97%.
No changes have been made to the general settings.
I'll keep investigating this issue further, as it seems it randomly occurs.
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MPY.