I always start my yoga classes with a breathing exercise, and a student recently asked me: “When you tell us to slow our breathing down, what is that actually doing?” Good question. I think the why matters just as much as the practice itself – understanding what’s happening in your body makes a tool such as a breathing exercise feel less like faith, and more like something you can actually rely on.
So here’s what’s going on, in simple terms.
Your stress response is older than you are
When your nervous system perceives a threat – a deadline, a difficult conversation, sometimes a long to-do list – your sympathetic nervous system switches on. Heart rate rises, breathing becomes quicker and shallower (i.e., high in the chest), and the body releases cortisol via the hypothalamic-pituitary-adrenal (HPA) axis. This system evolved to help you respond to genuine physical danger (think: sabre-tooth tiger), but nowadays we are rarely in real physical danger – the system still works the same way when we’re exposed to modern-day stressors like an overflowing inbox.
This system is meant to keep us safe, by triggering a quick reaction to get us into safety. The trouble is that it wasn’t designed to stay switched on constantly. Living with it activated for long stretches – through sustained pressure, ongoing stress or chronic illness – takes a real physiological toll over time.
The other half of the system
Fortunately, the body also has a built-in brake: the parasympathetic nervous system, largely run by the vagus nerve. This is the system responsible for rest, digestion and recovery, working in balance against the sympathetic “go” system. One way researchers measure how well this balance is functioning is heart rate variability (HRV) – essentially, how much the gap between heartbeats naturally varies. More variability tends to be a good sign, indicating a nervous system with greater flexibility to respond to challenges; persistently low HRV, by contrast, has been linked to poorer long-term health outcomes (Streeter et al., 2012).
Where breath comes in
This is where slow, deliberate breathing earns its place – not as a vague calming ritual, but as a direct lever on this system. A systematic review in the field of neuroscience drew together psycho-physiological research on slow breathing and found consistent links between slow-paced breathing and markers of parasympathetic activation across cardiovascular, respiratory and nervous system measures (Zaccaro et al., 2018).
A more recent meta-analysis – a study that pools results from many separate studies to build a more reliable overall picture – found that voluntary slow breathing reliably increased vagally-mediated HRV (the component of heart rate variability driven by the vagus nerve), both during a single practice session and across longer-term interventions (Laborde et al., 2022). And the effect doesn’t take long to register: one study found that even a single session of slow, deeper breathing measurably increased vagal tone and reduced self-reported anxiety in both younger and older adults (Magnon et al., 2021).
The effect isn’t limited to breath alone, though. A meta-analysis of 42 randomised controlled trials found that yoga practices involving physical postures were associated with reduced cortisol, lower resting heart rate, lower blood pressure and improved HRV compared with people doing a different, non-yoga activity (Pascoe et al., 2017) – suggesting it’s the broader practice, not just isolated breathing exercises, that supports this same regulatory system.
A practical takeaway
You don’t need an hour on a mat to access any of this. A simple extended-exhale breath – inhaling for a count of four, exhaling for a count of six to eight – is enough to begin nudging this system toward balance. Try it for two or three minutes: before a stressful meeting, after a hard conversation, or simply when your thoughts feel like they’re moving faster than you are.
It won’t eliminate stress from your life. But it’s one of the few tools backed by research that you can use anywhere, for free, in under five minutes – which is, I think, exactly what a practical toolkit should be.
References
Laborde, S., Allen, M. S., Borges, U., Dosseville, F., Hosang, T. J., Iskra, M., Mosley, E., Salvotti, C., Spolverato, L., Zammit, N., & Javelle, F. (2022). Effects of voluntary slow breathing on heart rate and heart rate variability: A systematic review and a meta-analysis. Neuroscience and Biobehavioral Reviews, 138, 104711.
Magnon, V., Dutheil, F., & Vallet, G. T. (2021). Benefits from one session of deep and slow breathing on vagal tone and anxiety in young and older adults. Scientific Reports, 11, 19267.
Pascoe, M. C., Thompson, D. R., & Ski, C. F. (2017). Yoga, mindfulness-based stress reduction and stress-related physiological measures: A meta-analysis. Psychoneuroendocrinology, 86, 152–168.
Streeter, C. C., Gerbarg, P. L., Saper, R. B., Ciraulo, D. A., & Brown, R. P. (2012). Effects of yoga on the autonomic nervous system, gamma-aminobutyric-acid, and allostasis in epilepsy, depression, and post-traumatic stress disorder. Medical Hypotheses, 78(5), 571–579.
Zaccaro, A., Piarulli, A., Laurino, M., Garbella, E., Menicucci, D., Neri, B., & Gemignani, A. (2018). How breath-control can change your life: A systematic review on psycho-physiological correlates of slow breathing. Frontiers in Human Neuroscience, 12, 353.
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