I've also used autosleep and it's nice. The Apple Watch however isn't made to track sleeping. Not that it can't, but just that the battery life is horrible and it's too much of a hassle to charge it at the right time if I can't charge it at night. Might be better with newer models, but I still have the first one and it won't last a full day anymore, so tracking sleep is out of the question unless I charge it several times a day

I have used Pillow and was pleased with it, but I found a new app that I like better. It can be added to the Apple Watch apps menu so you can start it there, but it relies on your iPhone to track your sleep. It's called Sleep Cycle.

It uses the iPhone mic to track your sleep by sound and when you set the alarm in it, it wakes you at just the right point in your natural sleep cycle so that you wake easier and never feel like you were hit by a truck when you wake. I love it!

I actually used both it and Pillow for a few weeks because I didn't believe it could track my sleep without motion input. But I was surprised to see that each night the comparison of the sleep cycles from each app matched almost perfectly. And I guess the real proof is that when I use it to wake, I feel better at wake up and all day long.

An added plus is that now I can charge my Watch while I sleep because I don't need it to track my sleep anymore.

I use Sleepmatic and it works great

I use Autosleep