Battery saving mode currently resumes full tracking before the Smart Wakeup period to find the best time for you to wake up so that tracking uses only a fraction of the battery consumption for the entire night. When the battery drops below your defined standby threshold (default: 10%), the battery saver mode will resume.
This is a companion discussion topic for the original entry at https://sleep.urbandroid.org/docs/faqs/battery_saving_mode.html