Hello lurchbird,
If the Sleep schedule is active, it should turn on Do Not Disturb to prevent these notifications. First, check to make sure you're not allowing the Activity app (where Stand notifications come from) to be allowed in Do Not Disturb or Sleep Focuses. This will be in Settings > Focus > [name of Focus] on the paired iPhone.
If the Activity app isn't allowed in any of the Focus settings, before your next night during your sleep schedule, restart your iPhone and the paired Apple Watch which can resolve temporary issues like this:
Turn iPhone on or off - Apple Support
Restart Apple Watch - Apple Support
If you'd like to see more customization options added for the stand notification, like the ability to set a specific schedule for it, you can make those suggestions to Apple through their feedback site: Product Feedback
Cheers!