How to keep sleep tracking active on my Apple Watch after getting up and going back to sleep?

We have a newborn at home – I frequently get up in the night and then go back to sleep. This is causing sleep tracking to stop at the time when I get up, even if I have no alarm set. It seems that if I'm up and moving for ~1 hour then it won't track any sleep after that, even though sleep focus mode remains on. I have a sleep schedule set up that covers the whole night with no alarm.

How can I get the Watch to resume sleep tracking after I've been up for a while? Ideally I'd like it to track any sleep until the focus mode is turned off.

[Re-Titled by Moderator]

Original Title: How to keep sleep tracking active after getting up and going back to sleep