I'm guessing, but the Watch determines standing via the gyroscope and accelerometer. If you move about while you're sleeping, it may detect and interpret that movement as standing. These sorts of measurements are always going to be a bit of guesswork by the algorhythms, so they will not always be accurate. I regularly get activity notices from my Watch when I've just been sitting; apparently it detects my arm movements while I'm typing.
If you see other significant inaccuracies while you're awake, then there may be a fault with the sensors. Try restarting your Watch and see if that improves things. If not, you may wish to take the Watch to an Apple Store or authorized service center and see if they can check it.
Regards.