Heart rate variability, a higher order analysis done on heart beat intervals, gives a good estimate of how stressed you’re. Here is how it gives a fair idea about stress:

To understand stress we need to understand a bit of our autonomous nervous system anatomy. It can be divided into two parts sympathetic and parasympathetic.

The sympathetic nervous system (SNS) kicks in when your body is stressed, it can be understood as our fight mode. It starts producing norepinephrine- a neurotransmitter, which makes the heart beat faster and shortens the intervals between heart beats, giving the muscles and the brain the burst of oxygen to help them fight the emergency.

The parasympathetic system (PNS), on the other hand, releases acetylcholine — another neurotransmitter that makes the heart beat slower and increases the intervals between heart beats, giving heart muscles enough relaxation time.

In healthy people, these two systems are in balance. This means their heart rate is more variable. A decline in heart rate variability, increased sympathetic nervous system activity, is a sign that the body is not coping with stress well and is having trouble adapting to external demands.

Stress reflects the activity of the sympathetic nervous system. It is based on an assessment of the average heart beat interval, the number of intervals, how much the intervals differ from one to the next, etc.