https://ouraring.com/ is pretty awesome. I've started sleeping more, at more consistent times, and consuming less caffeine and alcohol. $75 off - http://ouraring.com/partners/rohannatraj - doesn't benefit me, just a coupon I received myself.
Here's a project my spouse is working on to see whether it's possible to warn you when you're getting sick using the combination of resting heart rate (from Fitbit), temperature (from Oura) and sleep data: