Investigate how Sleep Trackers Work

This appears to be a good starting point:
http://www.nosleeplessnights.com/how-do-sleep-trackers-work
[How Do Sleep Trackers Work And Are They Reliable_.pdf](https://github.com/dwyl/sleep/files/995741/How.Do.Sleep.Trackers.Work.And.Are.They.Reliable_.pdf) (_PDF in case the article disappears_)

## Tasks / Todo
+ [ ] Do research into sleep tracking wearables, apps and devices
+ [ ] Write a summary of the existing research.

Please summarise your findings in the README of this repo. thanks! 🎉
(_if the research get's "too big" we can always split it out into a separate file e.g: `sleep-tracker-research.md` and link to it from the main README..._)

Start with the linked article and its PDF, then research sleep-tracking wearables, apps, and devices. Summarize the existing research in the repository README, or create sleep-tracker-research.md and link it from the README if the material is too large; the task is done when both checklist items are covered.

Written by the indexing model from the issue text.