Great explanation of HRV by By Lucas Salmins.

What is Heart Rate Variability?
Heart rate variability (HRV) is the variation or difference in the gap between one heartbeat and the next. When we think of our heart rate, we usually think of 60–90 beats per minute (bpm), which is the range for an average heart rate. This number is an average of the speed of your heart rate over one minute; your heart rate actually changes from beat to beat, speeding up when you inhale, and slowing down upon exhale.  A pulse of 60 bpm will be an average of a heart rate over a minute that could vary between 55 and 66 bpm. Unlike a clock that ticks at a constant rate, the bio-electric beat of the human heart is constantly changing.  Read the rest of the article here

http://primaleye.uk/heart-rate-variability-stress-workout-recovery/#comment-4133