The Brain–Heart Connection

"Balance" isn't only about the heart. The variability of your heartbeat (HRV) is also a window into how the brain keeps the body balanced. On this page, we'll walk slowly through the brain–heart connection with sources cited, and at the end we'll share how Feelmo works with it.

A Two-Way Loop

The brain and the heart aren't a one-way street — they are constantly in two-way conversation. At the center of this exchange is the vagus nerve (the main pathway of the parasympathetic nervous system), which reaches from the brainstem down to the internal organs.

Figure: top = the brain (prefrontal cortex PFC and amygdala), bottom = the heart. The green two-way arrows represent the exchange that runs mainly through the vagus nerve.

This loop has, broadly, two directions.

• Brain → heart (efferent): After appraising the situation, the brain applies the brake (parasympathetic) via the vagus nerve and the accelerator via the sympathetic nervous system, tuning both the tempo and the "wobble" of the heartbeat. When you're calm, the vagal brake works well, and the heart flexibly varies from beat to beat.
• Heart → brain (afferent): Signals from the heart and blood vessels travel back to the brainstem, giving the brain a clue about "what state the body is in right now." The brain reads the body's voice and feeds it back into its tuning — and this back-and-forth is the loop.

The neural network formed by the brainstem, hypothalamus, amygdala, insular cortex, and others is called the central autonomic network, and it integrates the balance of the internal organs, including the heart (Benarroch, 1993). It may help to picture HRV as capturing the "flexibility" that appears on the heart side of this back-and-forth.

The Neurovisceral Integration Model

The psychologists Thayer and Lane proposed the neurovisceral integration model, which links emotion regulation with autonomic and cardiac control (Thayer & Lane, 2000). At the heart of this model is a pathway in which the prefrontal cortex restrains overreactions from regions like the amygdala and calms the heart by way of the vagus nerve.

Imagine a tense moment when a body that had braced itself switches over to "it's okay, you can settle now" — the hand on the reins in that moment is thought to be this prefrontal cortex–vagus nerve–heart pathway.

• The better this prefrontal–heart pathway is working, the higher HRV (especially its parasympathetic component) tends to be (Thayer et al., 2009)
• A meta-analysis of neuroimaging studies has also shown that HRV is associated with activity in the prefrontal cortex and amygdala (Thayer et al., 2012)
• The model has since been organized into a framework that hierarchically integrates everything from perception to action (Smith et al., 2017)

That's Why We Look at "Balance"

Through HRV, what we're seeing isn't merely the motion of the heart, but the capacity of the brain and body to adapt to a situation in concert — in other words, balance. The fact that HRV tends to drop under stress and tends to return as you recover both have this brain–heart loop in the background (Thayer et al., 2009; Thayer et al., 2012).

That's why Feelmo doesn't thrust the raw HRV numbers at you as a "grade." Instead, it gently translates them into a state of balance — how balanced or how unsettled you are right now. What matters isn't scoring high, but coming to know your own waves and gradually learning how to find your way back.

How Feelmo Works With It

Against the backdrop of this brain–heart connection, Feelmo reads the HRV measured by your Apple Watch as a Balance score (0–100) — a gentle gauge of how settled the heart is right now.

• What it's built from: The Balance score is grounded in HRV, measurement quality, and movement from your own baseline. The exact internal recipe is not public.
• How to read it: Low = unsettled, high = balanced. But it's not a number to compare with other people — it's read as the difference from your own baseline. Feelmo overlays that deviation onto the gauge as an aura, so that "a little lower than usual / coming back up" can be seen at a glance.
• Not showing the number too directly: A score on its own can feel cold. So Feelmo translates the Balance score into the expressions of six companions (Genki, Odayaka, Futsuu, Fuan, Panku, and Tsukare — Lively, Calm, Steady, Uneasy, Frazzled, and Weary) and sets them quietly alongside as words of observation. It never declares anything for certain.

The detailed design of which indicators are weighted and how they're combined is the core of the proprietary Feelmo analysis logic, and not all of it is made public. For a more careful explanation of how it works, see the related pages below.

Related pages: Getting to Know HRV · The Science of Balance · How to Read Your Balance

References

1. Benarroch EE. The central autonomic network: functional organization, dysfunction, and perspective. Mayo Clinic Proceedings. 1993;68(10):988–1001.
2. Thayer JF, Lane RD. A model of neurovisceral integration in emotion regulation and dysregulation. Journal of Affective Disorders. 2000;61(3):201–216.
3. 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.
4. 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.
5. Smith R, Thayer JF, Khalsa SS, Lane RD. The hierarchical basis of neurovisceral integration. Neuroscience & Biobehavioral Reviews. 2017;75:274–296.