Your Breathing Changes a Week Before Your Period

We often hear that our hormones make us unpredictable, but the data tells us otherwise: its a subtle but shockingly consistent rhythm. To honour the theme of understanding our bodies, we analyzed ~2.5 million sleep and symptom records from 2,941 users. In this study we looked into what PMS actually looks like beneath the surface.

Premenstrual Syndrome (PMS)

PMS refers to the physical and emotional symptoms that occur in the luteal phase, typically 1–2 weeks before menstruation. Around 90% of women experience some degree of PMS. Mild symptoms are common and normal, but more severe forms (like PMDD) can be life‑altering.

To get the clearest picture possible, we only analyzed the weeks prior to the start of the menses. We compared luteal phase days with vs without PMS reporting, and used 30-day rolling Z-scores to compare users against their unique physiological baseline.

Sleep and Activity

Using linear mixed‑effects models, we found small but statistically significant (p<0.05) shifts in our sleep physiology and our activity as a proxy for our behavior.

PMS is often associated with:

Slightly higher heart rate at night
Slightly lower HRV
Small increases in sleep fragmentation and sleep latency
Small decreases in activity

These shifts show an increase in our sympathetic activation, a state of lower recovery and higher strain on the body. They are not dramatic changes but this lines up with how many of us feel like “I’m jut a bit off…”

The Clear Signal: Breathing Rate

One physiological metric stands out with a meaningful effect size: Nighttime breathing rate increases on PMS days (FDR‑adjusted p < 0.05, Cohen’s d > 0.2). Wearable data shows a higher breathing rate roughly from a week prior to our period until a week later on cycles where PMS is reported.

**Figure 1:** Nighttime breathing rate trajectory across the menstrual cycle. In red, trajectories where PMS was reported shows higher breathing rate a week before period start compared to non PMS cycles, in blue.

Subjective or Objective

No one understands our body like we do, sometimes the best intuition is listening to how we feel and not a recovery or and HRV score.

PMS days are associated with substantially higher reporting of emotional and physical symptoms. These are all large effects (|d| > 0.2). So while physiology is subtle, the emotional and physical symptom signal is very real.

image (1).png — **Figure 2:** Cohen d effect size of symptom tagging prevalence in PMS days compared to non PMS days. Emotional symptoms, in red show higher prevalence than physical symptoms in blue.

We also ran a direct comparison between PMS days and period days for cramps, and it turns out cramp reporting is higher on PMS days than period days. Why?

This might reflect our behavior when it comes to tagging, we accept periods come with cramps and don’t tag them. Or, some women might experience more cramps during PMS than periods. As with anything subjective, there are many ways to interpret this!

The Chocolate Craving

We had a little fun exploring our data, and besides linear models and statistical test we checked for simple correlations, right in front of our eyes. To no one’s surprise the highest correlation between user tags were PMS with sugar!

image (2).png — **Figure 3:** Correlations between PMS and other tags.

Conclusion

The data offers a powerful reminder: your intuition is often more sensitive than your wearable. Our analysis shows that while PMS feels "loud" emotionally and physically, it remains quiet on your wrist. Aside from a clear shift in nighttime breathing rate, most physiological markers, like heart rate and sleep stages, show only subtle, minor fluctuations. If your app says your recovery is "optimal" but you feel exhausted trust yourself.

Understanding these subtle rhythms gives you the leverage to work with your biology rather than against it. Whether you’re an athlete or just having a busy week, if your energy dips in the luteal phase, remember it’s a biological pattern. Listen to the signs and adjust for them!