I have noticed that when I go to MENU -> Settings -> Activity Tracking -> Sleep Pulse Ox, there are only ON/OFF options. However, flicking that switch seems to cycle between the three possible states: 24-hour/During sleep only/Off. The evidence is that the battery estimate changes between 5 days/10 days/20 days. However, the switch only shows ON/OFF and it is completely inconsistent with the actual setting. It could be both ON or OFF for any of the possible values.
I see the same thing in the Android app - there are only ON and OFF states, but instead there should be a choice between the three possible. At least there the setting makes more sense and it toggles between either "All on" or "All off".
Because of this I ended up changing the setting from the website, then synchronizing. The switch is still in an indeterminate state, but at least the setting is correct.
I am using the current stable firmware (v9.30).
I assume everyone else is having the same issue, but they have not realized it yet. That ON/OFF switch makes no sense at all and it would be nice to see it replaced with some sort of a 3-way switch or a submenu where I could choose between the possible Pulse Ox Tacking states.