Being able to regulate our emotions comes with a lot of advantages, like enhanced social functioning and maintaining mental health. Most emotions are elicited by the environment and are associated with different degrees of physiological arousal. This physiological effect is usually generated by the autonomic nervous system, which is divided in the sympathetic nervous system and the para-sympathetic nervous system. During times of stress, the sympathetic nervous system kicks in, resulting in, amongst others, an increase in heart rate. In periods during which relatively little stress is experienced, the para-sympathetic nervous system becomes more dominant and causes the heart rate to slow down. Changes between these two systems can occur rapidly and depends on the ability (or the flexibility) of the autonomic nervous system to alter the heart rate. This flexibility is also associated with emotion regulation.

Heart rate variability (HRV) is a measure used for assessing the interplay between the two subdivisions of the autonomic nervous system and represents the capacity for regulated emotional responding. HRV measures are derived from an estimated variation among different sets of temporally ordered interbeat intervals, as measured by an ECG. It is influenced by many factors, but the two most important factors are the influence of the autonomic nervous system on cardiac activity and secondly the autonomic nervous system regulation by the central autonomic network. Considering the first factor, the sympathetic nervous system has an excitatory influence on the heart beat and is mediated by neurotransmission of norepinephrine. The influence on the heart beat is relatively slow. The para-sympathetic nervous system is mediated by neurotransmission of acetylcholine and has a relatively quick influence on the heart rate. Breathing on its own has an influence on the heart rate as well. Breathing in produces an increase in heart rate, breathing out produces a decreased heart rate. This is entirely mediated by the para-sympathetic nervous system. When looking at the second factor, the central autonomic network supports regulated emotional responding by flexibility adjusting the bodily arousal according to situational demands. This network consists of the cortical, limbic and brainstem regions. The output of this network has a direct influence on the heart rate.

There are two important theories relating autonomic flexibility causally by HRV with regulated emotional responding. The first theory, the polyvagal theory, was introduced by Porges in 1997. Within this theory, human functions is understood in terms of acquired, genetically based characteristics that probably have aided in human survival or reproduction. According to this theory, the autonomic nervous system developed in three stages. First was a slow-responding, unmyelinated vagus nerve (the dorsal vagal complex) developed that caused people to freeze when threatened. After that, acquisition of the sympathetic nervous system caused people to be able to either fight or flight. Finally, a fast-acting myelinated vagus (ventral vagal complex) enables people to rapidly withdraw and reinstate the inhibitory influence on heart beat activity. It is said to mediate facial expression, listening and other social behaviours.

The second theory is the model of neurovisceral integration. This model relates emotional responding with HRV through a perspective of dynamical systems. All the processes involved in emotion (behavioural, cognitive and bodily) are considered subsystems of a self-organizing system. This model also views the central autonomic network as the commander of governing cognitive, behavioural and bodily elements into regulated emotion states by inhibiting other potential responses. HRV is therefore considered as a proxy for the ability of the central autonomic network to regulate emotional responses by inhibiting according to the context.

Both the polyvagal theory and the model of neurovisceral integration specify an important role for inhibition of autonomic arousal in emotional expression and regulation and maintain that HRV measures carry information about ones’ capacity for this aspect of regulated emotional responding. Differences between the two theories relate to the parts of the brain that are involved and the theoretical frameworks they rely on. The polyvagal theory relies on the evolutionary perspective, the model of neurovisceral integration on the dynamical systems perspective.

Studies have supported the associations between the HRV and regulated emotional responding, including coping. It is suggested that a higher HRV is associated with more adaptive coping strategies. Scientists using the model of neurovisceral integration state that anxiety disorders result from a breakdown in the inhibitory process of the central autonomic network, which causes a continual state of excessive worrying. Diminished HRV is associated with depressive states. This elicits the potential for clinical applications, including HRV biofeedback that might have positive results for mood and anxiety.