Your heart beats at varying intervals based on the state of your nervous system. If you are nervous, scared, or exercising your heart will beat regularly and fast. If you are relaxed your heart will beat at irregular intervals and more slowly.
This is how that works. Your Autonomous Nervous System is made up of two subsystems, the Sympathetic and Parasympathetic Nervous Systems. The Sympathetic Nervous System (SNS) is your accelerator. It raises your heart rate and pushes you into fight/flight mode. When you are in fight/flight mode your Heart Rate Variability (HRV) is low as your heart is pumping blood regularly and fast in order to get you through the immediate danger.
Your Parasympathetic Nervous System (PNS) acts as a brake on your SNS. When there is no danger, the PNS is braking your SNS, slowing the beats and the results is a lower heart rate and higher HRV. When this is happening you are resting and digesting.
The variability in your heart rate is the interplay of your body’s accelerator (SNS) and brake (PNS). As you start examining your HRV readings, it will be a bit awkward because low HRV numbers mean you are stressed and high HRV numbers mean you are relaxed. HRV does not read stress, it measures your heart’s variability and the measures are inversely correlated with stress. High variability is low stress, low variability is high stress.
Find out more by reading about RR Intervals.