Why does my Apple Watch Series 9 alert me to stand while sleeping?

I've started wearing my Watch at night to track my sleep. Despite both the watch and the paired phone showing in sleep mode, the watch goes off during the night telling me it's time to stand! Am I missing some other setting I need to turn off?

Watch series 9, WatchOS 11.4, iPhone 13 Pro iOS 18.4.1

[Re-Titled by Moderator]