Why does the stand reminder wake me in the middle of the night?

Do you get the stand ring achieved notification before bedtime?

If it was me I would schedule a do not disturb focus for sleep hours.

Actually I like to put mine on the night stand and rest my skin for 8 hours and get a decent sleep. Only need to do a sleep test once a month to check for abnormalities. But that's my choice, you should be able to get the settings right eventually.

Maybe unpair and set up as a new Watch and take it from there.

In the end You ought to reach out to Apple Support directly. You should be able to reach them here  Click here > Contact - Official Apple Support  or phone numbers  Click here > Contact Apple for support and service - Apple Support   or Click here > Genius Bar Reservation and Apple Support Options - Apple 

Are you wearing it in bed with no focus or sleep monitoring running?

Maybe on the night stand with no Wrist Detection set?

Something in your settings is not right.

Reduce your stand hour goal. Certainly no more than 12