Battery saving mode currently resumes full tracking before the smart wake up period in order to find the best moment for your wake up, so the tracking uses up just a fraction of the battery consumption for the whole night. If the battery would drop under your defined stand-by threshold (default: 10%) the battery saving mode will re-occur.
This is a companion discussion topic for the original entry at https://docs.sleep.urbandroid.org//faqs/battery_saving_mode.html