Sleep is probably when that data is most likely to be of use, and when it's best able to measure it. I have an unusual doctor arrangement, I showed her my data and she found it very interesting.
With my watch (so probably yours too) you can go into watch mode while an activity is recording, and you can take a manual Pulse Ox reading any time you like to in the widget. So if you're hiking in the Bugaboos, you can take a data point when you stop for pictures and lunch, still have data when it's relevant (because you're at altitude) without paying too much battery for it.