Your Garmin Knows Your Period Is Coming: Why HRV Drops Every Month

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 chart 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

MissingMCP is a free, open-source gateway between your Garmin Connect account and an AI assistant. It's hosted, so there's no terminal, no install, and no code.

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

That's genuinely the whole thing. No hacker wizardry — your Garmin password is used once to sign in and then discarded, only encrypted tokens are kept, and the entire project is open source if you want to check for yourself.

This is free, open source, and maintained by one person. If you end up using it regularly, buy them a coffee. Tools like this only keep existing when the people using them chip in. More on MissingMCP →

Step 2: paste this prompt

Claude fetches four months of HRV in 30-day chunks and builds an interactive chart you can scrub through on your phone.

Using my connected Garmin tools, chart my HRV for the last four months.

Fetch the data first, then build:

- Work out today's date and go back four months.
- Garmin HRV trend endpoints usually cap at 30 days per call, so split the
  range into 30-day chunks and make one call per chunk. Don't skip any, and
  don't shorten the range to fit a single call.
- Expect two numbers per day: the seven-night rolling average, and that
  night's highest five-minute reading. Some days may be missing — leave the
  gap and say so, rather than filling it in.

Then give me one self-contained HTML artifact:

- A line for the seven-night average and dots for the nightly peaks, both in
  milliseconds on a shared axis.
- A button that hides the peaks and rescales the y-axis to the average's own
  range, because the average looks flat when the peaks share the scale.
- Hover or touch to scrub: a crosshair plus that day's average, peak, and
  Garmin status (balanced, unbalanced, or low).
- A thin band under the chart colouring every night by its status.
- Summary figures up top: latest seven-night average, the min-to-max range of
  that average, how many nights were balanced, and the highest single night.
- Dark, calm palette, tabular numerals, readable on a phone, no external
  JavaScript libraries.

Finish by telling me what the data actually shows in two or three sentences.

Reading the chart

Look for a dip in the seven-night average that comes back roughly every four weeks, and check whether it lands in the week before each period. Because it's a rolling average, the dip shows up a few days later than on the individual nights, and the status band underneath will often turn unbalanced in those same stretches.

To make it explicit, give Claude your last few period start dates as a follow-up and ask it to mark them on the chart. If the dips line up with them cycle after cycle, you've found the same thing we did.

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.