Heart rate variability (HRV) has two histories. In the West it grew from a vicar’s observation about breathing, through physiology labs and cardiology wards, to a 1996 standard that every wearable still follows. In the Soviet Union it grew out of space medicine, a story told on HRV in Soviet space medicine. This page follows the Western line, mostly as George Billman set it out in his 2011 historical review (Billman, 2011).
Before the ECG: a pulse that follows the breath
Doctors have felt the pulse since antiquity, but they could not time it. Billman dates the first accurate pulse counts to 1707, when a watch with a second hand that could be stopped, the “physician’s pulse watch”, went on sale. With it, the Reverend Stephen Hales, an English clergyman and experimenter, noticed in 1733 that the pulse changed with breathing. That rhythm, the heart speeding up as you breathe in and slowing as you breathe out, is respiratory sinus arrhythmia Respiratory sinus arrhythmia The normal rise in heart rate as you breathe in and fall as you breathe out. It is driven by the vagus nerve and makes up much of resting HRV. More about Respiratory sinus arrhythmia , and it is still the largest part of short-term HRV.
Carl Ludwig, a German physiologist, recorded it in 1847 with his kymograph, a rotating drum that traced blood pressure and breathing onto smoked paper. Gernot Ernst’s history puts the probable discovery of HRV at that recording (Ernst, 2017). The electrocardiogram ECG A recording of the heart's electrical activity, taken with electrodes on the skin. ECG-based RR intervals are the reference standard for measuring HRV. More about ECG followed in 1895 (Billman, 2011), making each heartbeat a sharp spike on paper. For the next sixty years, though, measuring the gaps between thousands of beats by hand was more work than anyone had a use for.
The 1960s: computers and the unborn
Two things changed in the 1960s. Digital signal processing made it possible to measure every beat in a long recording, and doctors found a reason to care. In 1965, Hon and Lee reported that distress in unborn babies was preceded by changes in the gaps between heartbeats before the average heart rate changed. Billman counts this as the first clinical use of HRV. The idea that the variation carries information the average hides is the root of the whole field.
In the 1970s the same idea reached adults. HRV was used to detect damage to the nerves that control the heart in people with diabetes, and it was linked with survival after a heart attack (Billman, 2011). Both uses rested on a simple reading: a heart that no longer varied was a heart that had lost its nervous control.
1981 to 1987: the spectrum and the survivors
1981, the spectrum. Solange Akselrod and colleagues at MIT published a short paper in Science showing that power spectrum analysis of heart rate fluctuations could separate the nervous system’s contributions by frequency. 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 and 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 systems made frequency-specific contributions to the spectrum, and the renin-angiotensin system, a hormone system that controls blood pressure, shaped the slow peak near 0.04 Hz (Akselrod and colleagues, 1981). Billman lists it among the pioneering studies after which the field grew quickly. It is the origin of the LF LF power The variance of the heartbeat intervals that falls in the 0.04 to 0.15 Hz band, in ms²: rhythms repeating every 7 to 25 seconds. Once read as sympathetic activity, which is now disputed. More about LF power , HF HF power The variance of the heartbeat intervals that falls in the 0.15 to 0.40 Hz band, in ms²: the range of normal breathing. It mainly reflects vagal activity when breathing is 9 to 24 breaths a minute. More about HF power and VLF VLF power The variance of the heartbeat intervals in the 0.003 to 0.04 Hz band, in ms²: rhythms repeating every 25 seconds to 5 minutes. It needs at least a 5-minute recording and is best read over 24 hours. More about VLF power bands on your app’s advanced screen.
1986, the ratio. Massimo Pagani and colleagues in Milan analysed 57 healthy people lying down, tilted upright, breathing to a metronome and on beta-blockers, plus conscious dogs with their cardiac sympathetic nerves cut. They found two main peaks, high frequency near 0.25 Hz and low frequency near 0.1 Hz, and proposed the ratio of the two as a marker of sympathovagal balance LF/HF ratio LF power divided by HF power, with no unit. It was proposed as a gauge of balance between the sympathetic and parasympathetic systems, but research shows it cannot measure that reliably. More about LF/HF ratio : at rest about 3.6, during upright tilt about 21 (Pagani and colleagues, 1986). The LF/HF ratio became the most quoted and, later, the most disputed number in HRV; the glossary entry on LF/HF explains why.
1987, the survivors. Robert Kleiger and colleagues analysed 24-hour Holter tapes from 808 people who had survived a heart attack, recorded about 11 days after it. Of everything measured, HRV, defined as the standard deviation of all normal beat intervals, had the strongest link with death over the next 31 months. Those with SDNN SDNN The standard deviation of all NN intervals in a recording, in milliseconds. It measures overall variability, and its value depends strongly on how long the recording lasts. More about SDNN under 50 ms were 5.3 times as likely to die as those over 100 ms, and the link held after adjusting for how well the heart pumped (Kleiger and colleagues, 1987). This study made HRV a cardiology tool and is still the number people quote first; the page on HRV and longevity puts it in context.
1996: the standards
By the mid-1990s, hundreds of papers used dozens of definitions. In 1996 a task force of the European Society of Cardiology and the North American Society of Pacing and Electrophysiology published shared standards: the time-domain measures (SDNN, RMSSD RMSSD Root mean square of successive differences: an HRV metric built from how much each heartbeat interval differs from the next, in milliseconds. It mainly reflects vagal activity. More about RMSSD , pNN50 pNN50 The percentage of neighbouring NN intervals that differ by more than 50 milliseconds. It tracks fast, vagally driven changes, like RMSSD, but as a share rather than a size. More about pNN50 and others), the frequency bands ( ULF ULF power The variance of the heartbeat intervals below 0.003 Hz, in ms²: rhythms slower than one cycle every 5 minutes, including the day-night cycle. It can only be measured from a 24-hour recording. More about ULF power , VLF, LF, HF), recording lengths of 5 minutes and 24 hours, and the rule that only normal-to-normal beats count, with ectopic beats Ectopic beat A heartbeat that starts outside the heart's normal pacemaker and usually comes early. Ectopic beats are common, but they distort HRV unless removed or corrected. More about Ectopic beat removed first (Task Force, 1996). Ernst notes that since 1996 most measurements and calculations have followed it. When your ring reports RMSSD from the night, it is using a definition written for 24-hour hospital tapes.
Go deeper What the standards settled and what they did not
The Task Force settled definitions, units and recording rules, and it gave cautious readings of what each measure means: RMSSD and HF as mainly vagal, LF as a mixture with a baroreflex contribution, and the LF/HF ratio as a rough index of balance. It did not settle normal values for the general population, the meaning of VLF, or whether LF measures sympathetic activity at all. Those arguments ran through the following two decades, and the site’s glossary entries on LF power and HF power record where they stand.
After 1996: nonlinear measures, athletes and wearables
Nonlinear measures. Researchers borrowed tools from physics to describe the shape of the beat series rather than its spread: detrended fluctuation analysis DFA α1 The short-term scaling exponent of detrended fluctuation analysis. Near 1 the beats have memory at every scale; near 0.5 they behave like noise. α2 is the same slope over longer stretches. More about DFA α1 , entropy, Poincaré plots. In 2015 a joint position statement by European and Asia-Pacific cardiology groups reviewed them and concluded that, while they had deepened the technical understanding of the signal, their success in producing new clinical tools had been limited (Sassi and colleagues, 2015).
HRV-guided training. By the 2000s sports scientists were using HRV to watch how athletes coped with load. In 2007 a Finnish trial made the morning reading a decision: 26 moderately fit men were split into a fixed-plan group, an HRV-guided group that ran hard only when morning HRV was normal or rising, and controls. Both training groups got faster, and the HRV-guided group improved its peak oxygen uptake from 56 to 60 ml/kg/min (Kiviniemi and colleagues, 2007). The page on HRV and training follows that thread.
Wearables. Phone cameras, chest straps paired with apps, and then rings and watches brought a nightly HRV number to millions of people from the 2010s. Their accuracy varies: against a medical ECG, a strap-and-app combination was within about 4% on RMSSD and a ring within about 7%, while a phone-camera app was off by over 100% (Stone and colleagues, 2021). The measures are the Task Force’s; the sensors are new. How they differ is the subject of how HRV is measured.
The two lines meet
The Soviet line started separately in the early 1960s, with Parin and Baevsky using beat-to-beat intervals to judge how cosmonauts coped with stress, and produced its own measures such as the stress index Baevsky stress index A number built from the histogram of heartbeat intervals, with no unit. It rises as the rhythm gets steadier and less variable, which Baevsky read as strain on the body's regulation. More about Baevsky stress index . From the 1990s, joint Russian, American and German studies on Mir and the International Space Station brought the two traditions together, and Russian reviews cite the Western papers above. That story is told on HRV in Soviet space medicine.