iPhone battery draining overnight
Why does my iPhones battery drain overnight to half the percentage, so annoying
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It would depend of it anything is running overnight. Why in the world would you not have the phone connected to power and wi-fi overnight so that it is fully charged in the morning and then iCloud would have also been able to do its backup?
To troubleshoot the battery issue, go to Settings>Battery, and see what app(s) are using the highest percentage of battery, and start there.