There is already 3rd party Sleep Tracking Apps, but I believe Apple will take it to a new level. Not sure how the battery life will be, perhaps Apple will find away to conserve the battery when the feature is on.
I just don't see this happening yet. Of course I could be wrong, but even if they found a way to conserve battery there is no way they've figured out how to make the battery last for multiple days. The Series 4 battery actually is a little bit worse than the Series 3, so I think you are reaching a little bit to think that Apple will have a magical battery in their next watch one year later.