Health data question: With things like smart watches, calorie tracking apps and sleep cycle alarm apps - users are gathering a lot of data about their daily habits. While a lot of it might be quite dirty (e.g. sleep alarm apps may miss records for weekends, or automatic time guesses may be off, users may lie to calorie tracking apps) - in aggregate the data has to be somewhat useful, no?

Wondering:

1. Why there aren't any major projects asking for this data?
2. What are the risks/implications of using this data - even if donated freely by volunteers?
3. What could such data actually be useful for? (e.g. HR data for a million people over 5 years, OR sleep timing and quality for similar sets of people, OR complete food logs from fitness enthusiasts or chronic dieters)