When do you get a notification that the Sleep Index needs to be recharged?

When do you get a notification that the Sleep Index needs to be recharged?
Is there always one more night of battery left before the alert, or do you need to keep track of the bettery level yourself?

  • I get an automated notification that the battery level is low when the battery drops below 20%.  No need to check it yourself, although there is no harm in trying.  I keep checking the battery level daily for now because I am curious to know how much the battery drops per night.  For now I am getting around eight nights out of one charge.