Become a MacRumors Supporter for $50/year with no ads, ability to filter front page stories, and private forums.
If you don’t want to use the watch, I recommend Sleep Cycle.

I’ve been using the free 30 day trial. It’s not bad but I was hoping for something with a bit more detail. Plus despite its stating it won’t pick up sounds from anyone else it does heck it picked up birds chirping and said I was snoring.
 
Register on MacRumors! This sidebar will go away, and you'll see fewer ads.