The Apple Watch might include sleep tracking sooner than you think. Sources talking to 9to5Mac claim to have significant details as to how the health feature will work, with word that it might be announced "as early as next week" at the September 10th event. Some details aren't surprising, but they suggest this isn't just a throwaway addition.
The technology would rely solely on the watch's motion, heart rate and noise detection to gauge the quality of your sleep. You wouldn't need a Beddit sensor to fill in the gaps. This isn't shocking (other wearables have done this), but there would be a dedicated Sleep app on the watch in addition to rest quality data in the Health app. You'd get reminders to charge your watch before you tuck in for the night, a watch-only wake-up alarm (including vibration-only if you prefer), and you could even specify a designated watch just for bedtime.
The main unknown: whether or not you'll need new wristwear. Apple sometimes ties Watch features to new hardware and might have refresh devices on the way, but the sleep tracking as described doesn't sound like it explicitly requires an upgrade. This doesn't mean that anyone running watchOS 6 will get sleep tracking. It's just that existing users might not be in a rush to upgrade.