I don't know anything about gadgets designed specifically for sleep tracking, unfortunately. I only know of wearables like from Fitbit as you mentioned, and I personally have a Mi Band 2. I haven't used the sleep tracking function, but my friend's mom has, and she's quite happy with it. If you are only looking for something to track your sleep I don't think I can be of much help, but if you don't mind looking at fitness bands as well, I could recommend the Mi Band. Pretty cheap, and gets some great reviews.