While there are many heart rate apps available, Inner Balance is monitoring heart rate variability (HRV) - a measurement of beat-to-beat changes that occur within heart rhythms and a sophisticated and highly sensitive observation of emotional fluctuations.

ENGADGET: HeartMath unveils Inner Balance iOS app and sensor for managing stress, clearing the mind

They quantify your stress level by calculating your heart rate and the minute variations in the time between each beat, a measurement known as heart rate variability (HRV), which has been used to gauge the likelihood of mortality after a heart attack and to help assess certain psychiatric conditions.

WSJ: Keep Calm and Carry a Gizmo