What's Better For Improving My HRV: Exercise Or Sleep?

Modern sports science has come a really long way in helping us understand exactly why we’re doing what we’re doing when we start a new health routine, providing feedback that inspires training adjustments at a large and small scale. Where our parents' generation might have improved their 10K times simply by running faster for longer periods of time, modern wearables like heart rate trackers can help us dial in actionable data every day.

Today, we understand that cardio fitness largely comes down to how well our hearts can bounce back from the stresses we put them through. All of which begs the question, when it comes to improving our heart rate variability, are tough workouts actually best? Or is it actually the downtime we get each night while we sleep that helps us lock in and improve at the physiological level? 

We decided to ask some medical experts to weigh in on the whole "sleep versus exercise" debate. And the results that might surprise you.

What is HRV?

First, let's get a quick lay of the land on what, exactly, we are measuring here. Heart rate variability (HRV) measures the milliseconds between heartbeats. It’s a key indicator of the health balance between our ‘fight or flight’ (sympathetic) nervous system, and our ‘rest and digest’ mode (parasympathetic nervous system). In athletic terms, having a strong HRV score shows your heart is able to efficiently switch between stressed out (i.e. sprinting) to fully recovered – or vice versa – making it a key indicator of overall fitness.

“HRV reflects how adaptable the cardiovascular system is under changing demands,” explains Rajeev L. Narayan, MD, director of interventional cardiology for the Northwell Vassar Brothers Medical Center. (He’s also a big marathoner and triathloner, and enjoys Jiu Jitsu). “Higher HRV generally indicates the ability to buffer physiologic stress, while lower HRV can signal heightened systemic stress, inflammation, or impaired recovery.” 

The average adult's HRV ranges from 19 up to 75 milliseconds. The higher the number, the better your fitness.

What impacts HRV?

Dr. Narayan adds that key lifestyle and biological influences on our HRV include: 

• Sleep quality
• Exercise load
• Psychological stress
• Illness
• Alchol consumption
• Hydration
• Metabolic health
• Medications (like beta blockers)
• Underlying cardiovascular disease
  

How do you measure HRV?

Outside of a clinical lab setting, HRV is usually measured via wearables like those from Oura, WHOOP, and Apple Watch, or chest strap monitors like the Polar H10. 

Professor Pierre-Marc Bouloux, an authority on physiology and psychology at HOOKE, London, is a fan of WHOOP, putting its accuracy in measuring HRV at around 99%. 

Why is everyone obsessed with HRV right now?

We’re living through the Golden Age of Optimisation, and fitness is leading the charge. Alongside a genuine desire to feel and move better, understanding our HRV also helps drive performance – and who doesn’t like posting new Strava PBs?

Best of all, all this metric tracking means that we are now able to strap these high-tech gadgets to our wrists and chests at a pretty low cost, and next to zero intrusion. “The proliferation of wearables has brought a once research‑heavy metric into everyday life,” agrees Dr. Narayan.

He adds that although clinical interest in HRV is nothing new, the proliferation of wearable devices is actually helping diagnose patients sooner. “What is new is that [because they own their own devices] patients are arriving with lots of data, which can spark meaningful conversations about sleep, stress, and training habits,” he says.

How does exercise impact HRV?

Exercise can work wonders on both our mental and physical health, working at the biochemical level to make all sorts of changes that can improve our wellbeing. All you have to do is lace up some running shoes. 

“From a cardiovascular standpoint, regular exercise improves stroke volume, reduces resting heart rate, enhances nitric oxide signalling, and improves baroreceptor sensitivity — all mechanisms that support higher baseline HRV over time,” Dr. Narayan explains.

Dr. Narayan is such a fan of exercise that he calls it the “primary non-pharmacological intervention to improve physiologic autonomic outcomes” which, if you don’t have a doctorate in medicine, means it’s the best thing for your heart outside of medication. 

Overdo it, though, and you risk putting your heart at risk. “Excessive training without adequate recovery can produce sustained sympathetic activation and suppressed HRV,” Dr. Narayan warns, adding that HRV tracking devices are great for catching the early signs of overtraining. 

A peer-reviewed study in the Archives of Gerontology and Geriatrics, which monitored long-term physical activity levels, discovered that consistent moderate-to-vigorous exercise was the sweet spot for higher HRV, and overall health as we age.

How does sleep impact HRV?

When we sleep, our entire system gets a reset. And we've never been quite as aware of how important this biological function is as we are today. Dr. Narayan calls sleep “the primary daily period of autonomic recalibration,” and its impact on HRV is just as beneficial on things like our stress response and muscle repair. 

But not all sleep is created equal. Dr. Narayan points to a study in the journal Sleep Medicine Reviews, which evaluated the relationship between sleep and the heart and found that deep, REM sleep correlated with higher HRV, while sleep deprivation led to a persistent depression of HRV the following day. “In clinical practice, I often see poor sleep as a hidden driver of physiologic stress,” he says. “This is often more impactful than exercise patterns.”

So, to put it simply, sleep is a major factor in a healthy HRV, and when our sleep is fragmented or shortened, the sympathetic "fight-or-flight" system remains overactive and we don’t quite get the same benefits.

So, which one improves HRV the most?

There isn't a super straightforward answer here. Bouloux says it’s almost impossible to compare sleep and exercise in terms of one being better than the other for HRV, as the evidence simply doesn’t exist to draw a balanced conclusion. 

When interpreting our HRV, Bouloux says we should see it as a “rough and ready way of assessing one’s fitness and resilience over time.” He adds that intelligent training with enough time to recover, alongside adequate hydration, a reduction in alcohol consumption, and a healthy diet are perhaps the best things for our HRV long-term. It's not a one-and-done thing. It takes several different, healthy lifestyle habits to really move the needle.

“HRV is most useful when viewed as a trend reflecting overall physiologic load, rather than as a number to optimise obsessively,” says Dr. Narayan. He adds that both sleep and exercise are “foundational." And in many ways, we need both. The adaptations we build through exercise would not be cemented without adequate sleep. “I tell my patients that sleep restores the system nightly, while exercise helps to condition it over months,” he says. “For most individuals, improving sleep consistency is the fastest way to stabilise HRV, with exercise providing the longer‑term structural and physiologic benefits.”

In other words, balance is key. Exercise without rest won’t secure the improvements you want, and sleep without exercise will likely see your HRV remain static. 

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