Il tuo Garmin sa che sta arrivando il tuo ciclo: perché l'HRV cala ogni mese

Why her HRV drops every month

My wife's HRV drops every single month. For a while we blamed stress, a few bad nights or a hard week of training, but it kept landing at the same point, and we finally figured out why: it's her menstrual cycle. The dip shows up in the week before her period starts.

First: what HRV actually measures

Heart rate variability is the variation in time between your heartbeats. Your heart doesn't tick like a metronome: one gap between beats might be 0.85 seconds, the next 0.92. More variation generally means your nervous system is relaxed and in recovery mode. Less variation means your body is dealing with some kind of load. Garmin measures it overnight, while you sleep, when the readings are most comparable from night to night.

What happens across the cycle

After ovulation, the second half of the cycle starts: the luteal phase. Your body makes more progesterone, and that pulls the nervous system out of its rest-and-recover mode. On your Garmin, that looks like this:

• HRV goes down
• Resting heart rate usually goes up, often by a few beats

Then the period starts, hormone levels drop, and HRV bounces back up.

This isn't just one household's observation. Studies that measured hormones directly found progesterone was the hormone that moved with HRV: on days a woman's progesterone was higher than her usual, her HRV was lower than her usual, with the lowest values in the mid-to-late luteal phase.

So a lower HRV that week isn't automatically bad news

This is the useful part. If your HRV drops in the days before your period, that's a normal hormonal pattern, not necessarily poor recovery, overtraining or a cold on the way. Knowing that saves you from worrying about an HRV status that turns yellow like clockwork every month.

The pattern is personal, though. Some women see a clear dip, others barely any. Hormonal contraception suppresses the natural progesterone rise, so it often flattens or changes the pattern. Your own data is the only way to know what's normal for you. (If you want to know when an HRV drop is worth paying attention to, here are the warning signs.)

Why it's hard to spot in Garmin Connect

The HRV Status view in the Garmin Connect app shows 4 weeks at a time. A cycle is roughly 4 weeks too, so you only ever have one dip in view, with nothing next to it to compare against. On its own it just looks like a random bad week. The web version of Connect can zoom out further, but it still doesn't line your HRV up against your cycle.

The trick: have Claude look at 4 months at once

Four months gives you three or four full cycles side by side, and that's where the pattern becomes obvious. Here's how to get the full picture.

Step 1: connect Garmin to Claude

This uses MissingMCP, a free, open-source gateway between your Garmin account and Claude. No terminal, no coding.

1. Copy the MCP URL: https://missingmcp.com/garmin/mcp
2. In Claude, open your connector settings and add a custom connector with that URL
3. Sign in with your Garmin Connect account when prompted

Your password is used once to sign in and then discarded; only encrypted tokens are kept. It's maintained by one person, so if you end up using it, buy them a coffee. The full MissingMCP setup is here.

If you log your period in Garmin Connect's cycle tracking, Claude can read those dates directly. If you track it somewhere else, the prompt below simply asks you for the start dates.

Step 2: paste this prompt

Look at my last 4 months of Garmin data and show me whether my HRV follows my menstrual cycle.

## Step 1 — Pull the data

For every night of the last 120 days, pull:

1. HRV — overnight average, in ms
2. Resting heart rate — in bpm
3. Skin temperature change, only if my watch records it (skip it otherwise)

Then pull my period start dates from Garmin's cycle tracking. If there is no cycle data in Garmin, stop and ask me for my period start dates over the last 4 months before continuing.

Use whichever Garmin tools are available to you. Prefer tools that take a date range over one call per day. If a metric returns nothing or errors, mark it UNAVAILABLE and move on. Do not retry more than once.

## Step 2 — Line up the cycles

Split the data into cycles. Each cycle starts on a period start date (cycle day 1) and ends the night before the next one. Give every night its cycle day. Only analyse complete cycles.

For each cycle, compare two windows:

- Before: the 7 nights before the next period starts
- After: cycle days 5 to 11

## Step 3 — Show me

1. A line chart of nightly HRV across all 4 months, with every period start date marked as a vertical line. Put resting heart rate on a second axis.
2. A chart that overlays all cycles by cycle day, so I can see whether the dip lands in the same place each time.
3. A table with one row per cycle: average HRV and resting heart rate in the Before and After windows, and the difference.

## Step 4 — Explain it

In plain language: is there a consistent pattern? How big is the dip compared with my normal night-to-night variation? On which cycle day does HRV usually start dropping, and when does it recover?

Flag any cycle that doesn't fit, and point out anything else in the data that could explain it instead, like hard training blocks, illness, alcohol or travel.

This is pattern spotting, not a diagnosis. Don't draw medical conclusions.

What to do with the answer

Once you know where your dip usually lands, a lower HRV that week stops being a mystery. It also matters if you use an automated check like the daily illness score: a lower HRV plus a higher resting heart rate is exactly what that prompt looks for, so tell Claude your cycle day to avoid a false alarm every month.

One important note: your Garmin is not a medical device, and HRV is not a way to predict fertility or replace contraception. If your cycle or your symptoms change in a way that worries you, talk to a doctor. Cycle data is also personal, so only connect it if you're comfortable with an AI assistant reading it.