I have a Xiaomi fitness band 2. As far as I can tell, the sleep metrics it reports are pretty good. I wouldn't attempt to compare its accuracy to an Apple watch, but the Xiaomi's battery charge lasts about a week and it cost $30.
I'm really impressed with Mi Band 2 sleep tracking. It correctly matches the time I go to bed, and get up. Also gets each time I wake up during the night, or when I'm reading before sleep. I'm not sure about the "deep sleep" recording, and maybe I'm being suggested, but in nights with low deep sleep I fell terrible. Just monitoring my sleep I improved it. Sorry that it does not have a smart alarm neither an alarm to go to bed.
