This article pertains to: INFORM (V2)
Heart rate variability is the time between each of a person's heart beats.
There are some devices on the market that capture this data, and it is reported in the Validic Inform API for those sources that report it. Today, Fitbit and Emfit report this information.
Some sources, such as Garmin, may have devices that capture this data, but those vendors have not yet added the metric to their API.