Often because your heart is so slow. Heart rate variability (HRV) usually rises as resting heart rate Resting heart rate How many times your heart beats per minute while you are fully at rest. For adults it is usually 60 to 100 beats per minute, and lower in well-trained athletes. More about Resting heart rate falls, but only up to a point. When the vagal brake is strong enough to hold the heart at a very long beat interval, the beat-to-beat swings that 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 and high-frequency power 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 measure stop growing and can shrink. In 24-hour recordings from 76 healthy people, 35 showed this saturation, a plateau in high-frequency HRV once beat intervals got long enough (Kiviniemi and colleagues, 2004). Age and genes set much of the rest.
Saturation: when a strong brake reads as a low number
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 by releasing acetylcholine onto the pacemaker Sinus node The heart's natural pacemaker, a small patch of cells that starts every normal heartbeat. Left alone it fires about 100 times a minute; the vagus nerve holds it lower at rest. More about Sinus node cells. At moderate levels, each breath makes that release rise and fall, and the heart speeds up and slows down with it. At high levels the receptors are close to fully occupied, so the breathing-linked swing has less room to show. The heart stays slow and steady. A reading built from those swings, such as RMSSD, levels off or drops even though the brake is pressed harder than ever.
The Finnish study plotted 5-minute high-frequency power against the average beat interval across a day. In 35 of 76 healthy people the curve bent and flattened before their longest intervals, the signature of saturation. In 38 it kept rising. The authors warned that saturation can bias how vagal function is quantified from long recordings.
Martin Buchheit’s review of athlete monitoring describes the same thing in very fit endurance athletes: HRV can fall while resting heart rate also falls, and here the lower number comes with more 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 activity, not less. He recommends reading HRV next to heart rate for that reason (Buchheit, 2014). In elite endurance athletes, Plews and colleagues found both rises and falls in HRV going with poor adaptation, and recommended following each athlete’s own pattern over months with averaged readings (Plews and colleagues, 2013). The page on HRV vs resting heart rate explains the arithmetic that links the two.
Fitness explains less than you’d think
In morning readings from 28,175 app users, age, sex, body mass index and activity level together explained only about 15% of the differences in RMSSD between people, and age was the strongest of them (Altini and Plews, 2021). Two equally fit 40-year-olds can sit tens of milliseconds apart because of genes and the shape of their nervous systems. The population charts on normal HRV by age are a loose guide for exactly that reason, and the HRV that counts as good for you is the one inside your own usual range.
Measurement adds another layer. A morning reading in a chair, an overnight average from a ring and a 5-minute chest-strap recording lying down are three different quantities. A low number on one device is not a low number on another.
How to read your own data
- Read the pair. A low HRV with a low, steady resting heart rate and good sessions is the saturation picture. A low HRV with a rising heart rate, poor sleep and heavy legs is the fatigue picture. The two need different responses.
- Use your own range. Build a 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 the same way every day and judge the 7-day average against it, as the lesson on building your HRV baseline shows. Your range may sit below the population chart and still be normal for you.
- Watch the direction over months. Rising fitness over a season tends to show as a slower heart; the HRV trend may rise, plateau or even dip as saturation sets in. Neither is a reason to change training on its own.
- Note the device. If you switch from a strap to a ring or from mornings to overnight, start a new baseline.
For how training moves the number day to day, read HRV and training, and for the opposite puzzle, why is my HRV suddenly higher than usual?
When to talk to a doctor
A low HRV with a slow heart in a fit person is not a medical concern on its own. Talk to a doctor if a slow heart rate comes with dizziness, fainting, breathlessness or chest pain, or if you notice an irregular pulse. HRV cannot diagnose anything, and this page is not medical advice.