Age, Sex, and Individual Differences

What counts as "normal" for HRV (heart rate variability) varies enormously from one person to the next. Age, sex, constitution, the rhythm of daily life—many factors mean that even among healthy people, the numbers come out very differently. That's why Feelmo doesn't decide whether you're "high" or "low" by comparing you to someone else; instead, it watches over your own baseline. On this page, we gently explain why we do it this way, along with the scientific background and its sources.

Why "individual differences" matter so much

HRV is a value that captures the fluctuation in the intervals between heartbeats (R-R intervals). It quietly reflects the current state of your autonomic nervous system's balance.

But that "baseline level" really does vary a great deal from person to person. Even with the same lifestyle, one person's average might be another person's peak. That's why Feelmo's Balance score (0–100) should be read not as an "absolute passing line" but as the difference from your own usual state. Low means things are wobbling, high means things are settled—you look at that range of variation using your own measuring stick.

A gentle decline with age

HRV tends to decrease little by little as we age. A study covering an extraordinarily wide range of ages—from 10 to 99 years, nearly an entire lifespan—showed that HRV declines with age (Umetani et al., 1998).

This isn't a sign of illness; it's one of the natural changes that come with getting older. The important point is that a person in their 60s will naturally have lower numbers than someone in their 20s. In other words, it's meaningless to measure people of different ages against the same yardstick.

Figure: An illustration of how HRV declines gently with age, and how—even at the same age—individual differences (the width of the band) are large. The central line shows the average trend, and the pale band represents the spread of individual differences, i.e. "how much people scatter even at the same age." This figure is illustrative and does not show actual measurements.

Pay attention to the width of the band. The scatter within the same age can be far greater than the line (the average) itself. In other words, "where you yourself usually sit" is far more useful information than "the average value for your age."

There are differences by sex, too

HRV has been reported to come out differently depending not only on age but also on sex. A study examining short-term HRV in healthy people found differences by sex in several indicators (Voss et al., 2015). The reference values for HRV themselves differ by age group and sex—this is something review articles on HRV research have also laid out (Shaffer & Ginsberg, 2017).

The conclusion here is the same. The group averages for sex and age cannot pinpoint your own "state right now." That's precisely why the one you compare against is always "the you of yesterday and before."

So Feelmo compares you with yourself

• The same score means different things for someone in their 20s versus their 60s. Whether you're higher or lower than the person next to you doesn't mean much.
• What does mean something is the change from your own baseline. Feelmo always recommends "comparing with yourself" precisely because these individual differences are so large (Umetani et al., 1998; Voss et al., 2015).
• The balance gauge on the home screen is accompanied by a "vs. usual (±N)" badge and a range gauge that shows the deviation from your own baseline against your usual range. Rather than "above or below the general standard," it lets you see at a glance "how you're doing compared with your usual self."

How Feelmo handles these individual differences

Feelmo is designed from the very start with these large individual differences in mind.

The Balance score is built from the HRV read by your Apple Watch, centered on multiple HRV-derived clues (the fluctuation between one heartbeat and the next — the R-R intervals), together with measurement quality and how little you moved. These are interpreted not against a general standard, but in comparison with your own baseline. The same number can mean a good day for one person and a touch of tiredness for another—this is a mechanism for translating that difference using each person's own measuring stick.

Furthermore, whether a given habit (breathing exercises, for example) actually moves "your own" balance—rather than "somebody else's"—can be seen individually in the habit-impact analysis, Which habits help you. This is watching over the person right in front of us, not some average somebody.

How these indicators are combined into the Balance score lies at the heart of our proprietary Feelmo analysis logic, and the details are kept as a trade secret and not made public.

Related: Balance score · What Is HRV (Heart Rate Variability) · Heart Rate and Resting Heart Rate · Which Habits Help You.

References

1. Umetani K, Singer DH, McCraty R, Atkinson M. Twenty-four hour time domain heart rate variability and heart rate: relations to age and gender over nine decades. Journal of the American College of Cardiology. 1998;31(3):593–601.
2. Voss A, Schroeder R, Heitmann A, Peters A, Perz S. Short-term heart rate variability—influence of gender and age in healthy subjects. PLoS One. 2015;10(3):e0118308.
3. Shaffer F, Ginsberg JP. An Overview of Heart Rate Variability Metrics and Norms. Frontiers in Public Health. 2017;5:258.