@vietpq the watch cannot really see if your eyes are open or closed. It also does not really know if you are still partly dreaming or already fully awake. In reality sleeping is not a digital 1-0 state.
The watch only has measurement data of your pulse and of the watch movement.
If for example your pulse rises 1 hour before you actually are awake to a state to look at the time, and maybe you move your arm above a threshold amount and time, then the watch will have to conclude that you are awake.