The Brain–Heart Connection

The brain and cardiovascular system continually exchange signals. Heart rate variability (HRV) — the variation in time between heartbeats — can be useful in studying part of this connection, but it is not a scan of the brain and cannot reveal a person's thoughts, emotions, or ability to cope.

The main point

HRV is the variation in time between heartbeats. Brain and autonomic pathways help shape that variation, alongside breathing, blood-pressure reflexes, posture, activity, age, health, and measurement conditions. HRV is therefore an indirect and context-dependent signal, not a direct measure of "brain–body balance."

What is the vagus nerve?

The vagus nerve is a major pathway of the parasympathetic nervous system, the branch of the autonomic nervous system involved in functions such as slowing the heart. Respiratory cardiac vagal modulation — the breathing-linked parasympathetic influence on the heart — contributes to some HRV measures under controlled conditions.

That does not mean that HRV measures the strength, health, or "quality" of the vagus nerve. Current HRV guidance also cautions against using HRV as a specific measure of sympathetic nerve signals sent to the heart or of a single sympathetic-to-parasympathetic balance (Carter et al., 2026).

A two-way network

The connection is not a single wire between the brain and heart. Commands travel from the brain to the heart through parasympathetic and sympathetic pathways, while sensory information from the heart and blood vessels returns through vagal and spinal pathways.

Figure: a simplified schematic of two-way signalling. The arrows represent several neural and cardiovascular pathways; they are not an HRV measurement or a complete anatomical map.

The central autonomic network is the name used for interacting brain regions involved in regulating internal organs, including the heart (Benarroch, 1993). HRV records variation at the heart; it does not identify which brain region or pathway produced a particular change.

What is the neurovisceral integration model?

The neurovisceral integration model is a scientific framework linking brain networks involved in self-regulation with autonomic control of the heart (Thayer & Lane, 2000). Reviews and neuroimaging studies have reported group-level associations between some vagally mediated HRV measures and activity in regions including the prefrontal cortex and amygdala (Thayer et al., 2009; Thayer et al., 2012).

The model helps researchers form and test hypotheses. It does not establish that a high reading means someone's "thinking brain is in control," or that a low reading means poor emotion regulation. Association at group level cannot determine an individual brain state from one wrist measurement.

How does stress relate to HRV?

Across groups, psychological stress has often been associated with lower values of some HRV measures. There is substantial overlap between people and situations, however. Breathing pattern, movement, posture, sleep, exercise, medication, illness, age, and measurement method can produce similar changes.

For one person on one day, HRV cannot tell whether a change came from stress, recovery, excitement, breathing, or another factor. It also cannot classify a feeling as positive or negative.

How Feelmo uses this background

Feelmo summarizes available HRV-derived information and patterns in your own past into a proprietary Balance score (0–100). The processing details are not public.

The score is a product-specific wellness display, not a published measure of brain function or autonomic balance.
"Settled," "unsettled," and the six companion names are interface labels. They are not detected emotions, fatigue states, or diagnoses.
Comparing your own trend may be easier to understand than comparing raw values with other people, but a personal baseline does not prove what caused a change.
The app's score and labels are not validated by the brain–heart studies cited on this page.

About Apple Watch measurement

Apple Watch uses an optical pulse sensor, and HealthKit samples are not a continuous clinical electrocardiogram. Movement, sensor contact, recording length, breathing, watch hardware, software, and the intermittent timing of samples can affect the result. Sleep may reduce large movement, but sleep stages, awakenings, breathing, posture, and sampling still vary each night.

Important

HRV cannot determine a specific brain state, emotion, stress level, or disorder in an individual. Feelmo is not a medical device and does not diagnose or treat any condition. If symptoms concern you, consult a qualified healthcare professional.

References

Carter JR, et al. Guidelines for rigor and reproducibility of heart rate variability within human cardiovascular research. American Journal of Physiology-Heart and Circulatory Physiology. 2026. doi:10.1152/ajpheart.00041.2026. PMID: 42495990.
Benarroch EE. The central autonomic network: functional organization, dysfunction, and perspective. Mayo Clinic Proceedings. 1993;68(10):988–1001.
Thayer JF, Lane RD. A model of neurovisceral integration in emotion regulation and dysregulation. Journal of Affective Disorders. 2000;61(3):201–216.
Thayer JF, Hansen AL, Saus-Rose E, Johnsen BH. Heart rate variability, prefrontal neural function, and cognitive performance: the neurovisceral integration perspective on self-regulation, adaptation, and health. Annals of Behavioral Medicine. 2009;37(2):141–153.
Thayer JF, Åhs F, Fredrikson M, Sollers JJ, Wager TD. A meta-analysis of heart rate variability and neuroimaging studies: implications for heart rate variability as a marker of stress and health. Neuroscience & Biobehavioral Reviews. 2012;36(2):747–756.
Smith R, Thayer JF, Khalsa SS, Lane RD. The hierarchical basis of neurovisceral integration. Neuroscience & Biobehavioral Reviews. 2017;75:274–296.

About the cited research

These studies describe general models and group-level associations. They do not validate the Feelmo app, its score, or an individual interpretation of HRV.