One thing leads to another and fitness trackers for sleep monitoring is no exception. The Simple Dollar post showed up referring to an app that tracks sleep via your phone’s accelerometer and microphone without needing additional hardware: A Simple Tactic for Improving Sleep That 84% of People Miss by Drew Housman.
The post has some good information about sleep and its health effects as well as a few ideas for sleep monitoring. The app cited is Sleep Cycle alarm clock with free and premium versions. The app website also has some interesting reading.