You can reduce vibration zone in app Settings to “upper right” or “mixed”:
Or reduce vibration intensity in Watch’s Settings->Sounds & Haptics:
Just press Stop button
Or kill the app. Press watch button under Digital Crown on side -> swipe LeftReflux app to left -> press X
First, check that watch did vibrate over the night. Long periods of vibrations like on image indicates that there were vibrations but they where easy to ignore, not intense enough. In this case the best solution is:
Second, if there are only few vibrations on graph, then most probably it means that wider vibration zone is needed. Go to app’s Settings and select “full vibration zone”. In this case have vibration in most positions on right side and in few positions on back when hand is placed on abdomen.