The autonomic nervous system (ANS) is the portion of the nervous system that controls the body's visceral functions, including, but not limited to action of the heart, movement of the gastrointestinal tract and secretion by different glands, among many other vital activities. It is well known that mental and emotional states can affect visceral function by the ANS. Many research studies have examined the influence of emotions on the ANS utilizing the analysis of heart rate variability, or heart rhythms, which serves as a dynamic window into autonomic function and balance. While the rhythmic beating of the heart at rest was once believed to be monotonously regular, it is now known that the rhythm of a healthy heart under resting conditions is actually surprisingly irregular. These moment-to-moment variations in heart rate are easily overlooked when average heart rate is calculated. Heart rate variability (HRV), derived from the electrocardiogram (ECG), is a measurement of these naturally occurring, beat-to-beat changes in heart rate.

Various models propose that HRV is an important indicator of both physiological resiliency and behavioral flexibility, reflecting the individual's capacity to adapt effectively to stress and environmental demands. It has become apparent that while a large degree of instability is detrimental to efficient physiological functioning, too little variation can also be pathological. An optimal level of variability within an organism's key regulatory systems is critical to health. This principle is aptly illustrated by a simple analogy: just as the shifting stance of a tennis player about to receive a serve may facilitate swift adaptation, in healthy individuals, the heart remains similarly responsive and resilient, primed and ready to react when needed.

The normal variability in heart rate is due to the synergistic action of two branches of the ANS (the sympathetic and parasympathetic branches), which act in balance through neural, mechanical, humoral and other physiological mechanisms to maintain cardiovascular parameters in their optimal ranges and to permit appropriate reactions to changing external or internal conditions. In a healthy individual, thus, the heart rate estimated at any given time represents the net effect of the parasympathetic (vagus) nerves, which slow heart rate, and the sympathetic nerves, which accelerate it. These changes are influenced by emotions, thoughts and physical exercise. Our changing heart rhythms affect not only the heart but indirectly also the brain's ability to process information, including decision-making, problem-solving and creativity. They also directly affect how we feel. Thus, the study of heart rate variability is a powerful, objective and noninvasive tool to explore the dynamic interactions between physiological, mental, emotional and behavioral processes.

The mathematical transformation of HRV data is used to discriminate and quantify sympathetic and parasympathetic activity and total autonomic nervous system activity, reducing the HRV signal into its constituent frequency components and quantifies the relative power of these components.

Heart rate variability is a measure of the beat-to-beat changes in heart rate.

To summarize:

• Thoughts and even subtle emotions influence the activity and balance of the autonomic nervous system (ANS).
  
• The ANS interacts with our digestive, cardiovascular,immune and hormonal systems and is therefore ideally suited to translate mind states into organ functions/dysfunctions
• Negative reactions create disorder and imbalance in the ANS.
• Positive feelings such as appreciation and a state of relaxation create increased order and balance in the ANS, resulting in increased hormonal and immune system balance and more efficient brain function.

It has been shown in a number of studies that during mental or emotional stress and physical stress, there is an increase in sympathetic activity and a decrease in parasympathetic activity. This results in increased strain on the heart as well as on the immune and hormonal systems. Increased sympathetic activity is associated with a lower ventricular fibrillation threshold and an increased risk of fibrillation, in contrast to increased parasympathetic activity, which protects the heart.