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Such an api sort of already exists in the form of Fitbit's api which can be found at http://dev.fitbit.com/.

Fitbit holds a lot of the data hostage, but it's at least something that anyone wearing a Fitbit can easily grant access to.

What data do they hold hostage?

It was on HN[0] a few months ago that they don't give you access to minute by minute data, only daily totals.

Ah; FYI the API now supports Intraday Time Series fetching.

See https://wiki.fitbit.com/display/API/API-Get-Intraday-Time-Se... for more information. You can retrieve data in 1 min or 15 min increments.

you still cannot get access to the raw data or data in real time.