They do, it's called the Apple Watch. I find if I charge it in the morning and night while I'm brushing my teeth, the battery lasts all day. And there's a few apps that do good sleep reporting. I like AutoSleep and SleepWatch

What's the next thing you'll try to improve your sleep?

so there are several levels to "improving your sleep". I agree that if you sleep like shit and only get 4 hours per night you don't need the ring. you need to get more sleep.

but for some people that actually sleep well and want to tweak the quality of their sleep it can be valuable. let me give a few example of such tweaks: altering the time you go to bed, altering the surface you sleep on, reducing the amount of blue light you get, playing with the temperature in the room, not using screens X hours before going to bed, not eating X hours before going to bed. I could go on. The point I'm trying to make is that if you already get enough sleep (or you think you get enough sleep) the ring might help you. yes - the price is ridiculous. but it's not about the price, it's about improving a [very] important aspect of your life.

If you want to be scared shitless into paying attention to your sleep I recommend: "Why We Sleep" by Matthew Walker. Everything from cognitive abilities, fertility, risk of cancer and other diseases can be influenced by poor quality sleep. Sleep, along with eating properly and exercise is a pillar for a good quality life. I believe it's worth making it a priority.