iPhone battery draining overnight
Why does my iPhones battery drain overnight to half the percentage, so annoying
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.