Hi
For the purpose of crediting the hourly Stand achievement, your watch aims to identify when you have stood up and moved around for at least one minute during that hour.
Apple Watch uses an accelerometer to identify movement. It is possible that your arm movements during the night are sufficient to cause progress to be credited.
The following steps may help:
Calibrate your watch (follow the instructions in the linked article below). Calibration enables your watch to more accurately estimate various fitness and activity-related results. It does this by teaching your watch how your arm moves when you are walking and/or running:
Activity estimations also depend on your personal information. To check that this is accurate (and to update it over time):
- On your iPhone, in the Watch app, go to: My Watch > Health > Edit (upper-right).
Restart both your iPhone and your watch:
- Close the Watch app and Activity app on your iPhone:
- Double-click the Home button, then swipe up on each app preview to close it.
- Turn both devices off together, then restart your iPhone first:
More information:
Use the Activity app on your Apple Watch - Apple Support
