Perimenopause fatigue and HRV: what a low reading isn’t

Perimenopause fatigue and heart rate variability are related, but far more loosely than the tracking apps suggest. HRV genuinely moves with sleep, alcohol, illness, breathing and autonomic activity. What it does not do is keep score of how tired you are. In day-to-day wearable data the association between a person’s own HRV and their own reported energy is thin — and the most fixable cause of midlife exhaustion never reaches the wrist at all.

Why the number and the tiredness keep disagreeing

Two recent studies are worth holding side by side, because together they describe the situation better than either does alone.

In a 14-day study of healthy adults wearing a chest sensor, higher daily HRV was associated with better reported sleep, lower stress and lower fatigue, and the associations held after adjustment. Real signal, real direction.

Then a three-month study of a commercially available wrist tracker looked for the same thing in ordinary life and largely did not find it: self-reported stress and nervousness showed no significant association with the HRV the device recorded.

Both results can be true at once, and the reconciliation is the point. A relationship that is visible across a group, under careful measurement, can be far too small to read on your Tuesday. Averaged over enough people, tired days do run slightly lower. Inside one person’s week, the noise — measurement timing, posture, breathing, a late meal, where you were in your cycle — is comfortably larger than the effect.

There is also a quiet mismatch of units underneath all of this: Apple Watch reports SDNN, while most of the research and most of the charts people compare themselves against report RMSSD. We unpacked that in what a “normal” HRV over 40 actually means. Two different arithmetic operations on the same heartbeats do not produce comparable numbers.

The baseline you are measuring against is itself moving

This is the part almost nobody writes about, and it is the most useful thing here.

Researchers took 56 women from the Seattle Midlife Women’s Health Study — each with more than 60 rounds of health diaries — and analyzed stress and fatigue as dynamic systems rather than as symptom counts. Across the transition, fatigue became less stable over time, while stress generally became more stable, and the way the two moved together shifted from stage to stage.

Read that again with a tracking app in mind. Comparing this week against a rolling 90-day average assumes you have a settled resting point to return to. That study says the resting point for fatigue is precisely the thing loosening during the transition. A long average quietly averages across two different versions of you.

So the comparison that survives is a short one: these two weeks against the two before them. Not this month against your year, and certainly not against anyone else’s numbers. This is the baseline-not-average idea doing real work — it is not a slogan about individuality, it is a statement about which windows are still comparable.

A dip can belong to the night, not to your energy

HRV has other reasons to move, and some of them happened while you were asleep.

In midlife women monitored with ECG, cardiac vagal control changed during physiologically measured vasomotor events — including events that occurred during sleep. A night sweat you slept through can leave a mark in the overnight record while leaving no memory at all.

Fragmented sleep does the same thing from a different direction. Across cohorts, disturbed sleep is reported by roughly 16–42% of women before the transition and 39–47% during it, and nights break in a specific way during perimenopause — waking, not failing to fall asleep.

Which means a lower HRV morning may be a faithful record of last night’s events rather than a verdict on today’s reserves. The signal is doing its job; the interpretation people bring to it is the part that overreaches.

The cause your wrist cannot see

Here is the finding that changed how we think about this whole topic.

In 2025, SWAN investigators reported that about one in three women in the menopause transition experienced episodes of abnormal uterine bleeding. Women reporting three or more heavy bleeding episodes in the previous six months were more likely to feel tired (odds ratio 1.62) and more likely to feel worn out (1.44) after adjustment. The authors were careful: iron status was not measured in the cohort, so they could not connect the fatigue to anemia directly — but heavy menstrual bleeding is a well-recognized route to iron deficiency, and iron deficiency is one of the few causes of fatigue that is both common and straightforward for a clinician to assess.

No wrist sensor measures ferritin. Not HRV, not resting heart rate, not sleep stages. If your tiredness has lasted for weeks and does not track your bad nights — and especially if your periods have become heavy or prolonged — that is a conversation and a blood test, not a tracking project. Thyroid function sits in the same category.

How Perigee reads it

We show your own recent band rather than a population range, because for a signal this noisy the only stable reference is you. HRV appears next to sleep, resting heart rate and wrist temperature on the same dated timeline, and anything you log — tiredness included — sits beside it rather than being folded into a single number. There is no readiness figure, because we do not think one honest number exists here.

What the app is good for is the pattern nobody can reconstruct from memory: whether your flat days followed your broken nights, or arrived independently of them. That second pattern is the one worth carrying into an appointment. You can read more about what we do and do not claim and about the evidence behind each signal.

One small thing

For the next two weeks, write down one number each morning: how tired you feel, 1 to 5, before you look at anything on your watch. Then note whether the night felt broken.

At the end, look only for the mismatches — the tired mornings that followed unremarkable nights. Those are the days worth mentioning when you bring your data to an appointment, because they are the ones that point somewhere other than sleep.