Why my battery is discharging overnight on ultra power saving mode?

[danutnarian19]

Yesterday at 11:45 P.M I had 97% battery.When i woke up at 8:30 A.M my battery was 89%.All night,the phone was put on Ultra power saving mode.I did not use the phone until i woke up.I have a GS7.

 

[Rukbat]

The phone was still running, using power. 9% for 8 hours is not bad, that's only 1%/hour.