Stress usually lowers your heart rate variability (HRV) while it lasts, because the part of your nervous system that slows the heart steps back. But HRV only shows that your body is under some kind of load. It cannot tell you what is causing it, or how stressed you feel.
How stress changes your heartbeat
Two branches of the nervous system steer your heart. The parasympathetic Parasympathetic nervous system The "rest and digest" branch of the autonomic nervous system. It slows the heart, mainly through the vagus nerve, and drives most of the beat-to-beat variation in HRV. More about Parasympathetic nervous system branch, working mainly through the vagus nerve Vagus nerve The main nerve of the parasympathetic nervous system, running from the brainstem to the heart, lungs and gut. Its fast signals to the heart drive most beat-to-beat HRV. More about Vagus nerve , slows the heart and lets the time between beats vary more. The sympathetic Sympathetic nervous system The "fight or flight" branch of the autonomic nervous system. It speeds up the heart and makes it pump harder, but it acts too slowly to drive beat-to-beat HRV. More about Sympathetic nervous system branch, the “fight or flight” side, speeds the heart up and makes the rhythm more even. The lesson on the nervous system behind HRV explains this in more detail.
When you face a threat, a deadline or a hard conversation, the parasympathetic brake eases off. Your heart speeds up a little and the beat-to-beat changes shrink, so HRV goes down. Once the stress passes, the brake comes back on and HRV recovers.
A 2018 review of 37 studies found that HRV changed in response to stress in most of them (Kim and colleagues, 2018). The most common finding was lower parasympathetic activity, seen as a drop in high-frequency HRV. Brain imaging studies in the same review suggested a link between HRV and areas at the front of the brain that help judge how threatening a situation is.
What the studies show
Short-term stress in the lab
In a Danish study, 12 women did a standard computer task on two occasions: once with extra mental stressors added and once without (Hjortskov and colleagues, 2004). High-frequency HRV dropped during the stressful session. During 8 minutes of rest afterwards, HRV partly recovered, but blood pressure had still not returned to resting levels. The women’s own ratings of stress hardly differed between sessions, even though their HRV did.
Everyday stress
Real life is messier. A Finnish study measured 99 employees (59 men and 40 women, average age 40) at home (Hynynen and colleagues, 2011). Before one night of recording, each person filled in a stress questionnaire. The researchers recorded their heartbeats overnight and during a short lying-then-standing test after waking.
People who reported more stress symptoms had lower HRV in the morning test, by roughly 20% to 50% lying down and 30% to 75% standing. Their overnight HRV, however, was no different from the low-stress group’s, and neither were the stress hormones measured in their overnight urine. The high-stress group did show a bigger fall in HRV from the night to the morning test.
What HRV can’t show
- The cause. Alcohol, poor sleep, a cold or a hard workout can lower HRV just as a stressful day can. Your number does not know the difference.
- Good stress from bad stress. A hard training block and a work crisis can both push HRV down.
- How you feel. HRV and your own sense of stress do not always line up, as both studies above showed.
- A diagnosis. HRV cannot diagnose anxiety, burnout or depression. For more on anxiety, see does anxiety lower HRV?
Many devices turn heart rate and HRV into a stress score. Each maker has its own formula, so a “high stress” reading on one device may not match another’s.
How to read stress in your own data
- Tag stressful days. Note deadlines, conflict, travel or bad news in your app’s tags or notes. Over a few weeks you will see whether these days move your number.
- Compare with your baseline Baseline Your own usual HRV level and range, built from weeks of readings taken the same way. Comparing a reading with your baseline says more than comparing it with other people. More about Baseline . A single low day means little. HRV staying below your usual range for a week or more is more telling. The lesson on building your HRV baseline shows how to set one.
- Rule out the usual suspects. Before blaming stress, check for alcohol, late caffeine, short sleep or the start of an illness.
- Measure the same way each time. Readings taken during a stressful moment tend to be lower than calm, seated ones. If you take spot readings, take them at the same time and in the same position.
- Weigh it with how you feel. If your HRV is low and you also feel stretched, that is a reason to rest or slow down. If it is low and you feel fine, it may be noise.
If you want to try slow breathing, the pacer below sets a pace of six breaths a minute. What breathing exercises can and cannot do for HRV is covered in can breathing exercises raise HRV?
When to talk to someone
If stress, worry or low mood is affecting your sleep, work or relationships for more than a couple of weeks, talk to a doctor or a mental health professional. That advice holds whatever your HRV says.