[iPhone God App] It was found using the Heart Rate Variability Logger actually

Heart Rate Variability Logger lets you record, plot and export time and frequency domain Heart Rate Variability Features (includes experience sampling, rr-interval correction, comparison between recordings, activity monitoring & step counting, location tracking).

Main features:
- Extracts, plots, stores and exports heart rate, rr-intervals, time and frequency domain heart rate variability features (AVNN, SDNN, rMSSD, pNN50, LF, HF, LF/HF)
- Configurable experience sampling for events annotation
- Comparison between up to 3 recordings lets you get a better understanding of autonomic regulation and sympathetic/parasympathetic activity under different contexts in a glance
- Activity tracking: step counter or accelerometer derived motion intensity for user context (step counter only for iPhones 5S)
- Location tracking, either using GSM/WiFi networks (low battery consumption) or GPS (high accuracy)
- Configurable time window for features computation (choose between 30 seconds, 1, 2 or 5 minutes)
- RR-intervals correction can be enabled to prevent ectopic beats or artifacts from affecting HRV features
- Filters data based on the selected threshold
- Shows HR in the app icon
- Data export using iTunes or Dropbox
- Tested for recordings longer than 24 hours

Heart Rate Variability Logger requires a Bluetooth Low Energy (also called Bluetooth Smart or BLE or 4.0) heart rate sensor (tested with: Polar H7, mio alpha and Armour39)

More details are available here: http://www.marcoaltini.com/2/post/2013/12/heart-rate-variability-logger-app-details.html