Most cycle apps present day-by-day recommendations with a confidence the science doesn't support. We made a different choice — and we want to explain it.
A lot of apps will tell you exactly what to do on day 14. It feels personalized. It isn't — it's a population average applied to you as if you're average.
The research is clear: individual variability across the menstrual cycle is large. Two women on the same cycle day can have completely different hormone levels, energy, and performance capacity. Studies consistently show that group averages mask the real-world range of individual responses.
During beta, Wren shows cycle day only as a calendar estimate. Estimated phase does not change nutrition targets, Fuel Score, training, Next Best Move, Coach responses, or menus.
This is a deliberate boundary. Day-level recommendations would feel more personalized, but the science is not there yet to say day 8 is meaningfully different from day 9 for most women.
Symptoms and confirmed bleeding that you log may support Today's existing capacity and recovery guidance on your device. Deeper cycle-aware personalization is deferred until the model and intended use receive appropriate health review.
Every recommendation in Wren traces back to a source. Our training and nutrition guidance is anchored in peer-reviewed exercise physiology and women's health research — primarily the work of Dr. Stacy Sims and the studies below.
A full traceability document maps every user-facing recommendation to its evidence source. We update it when the science evolves.
The evidence on whether menstrual cycle phase meaningfully affects exercise performance is genuinely mixed. McNulty et al's 2020 meta-analysis found conflicting results with no clear consensus. We reflect that uncertainty in our language — "may," "can," "for many women" — rather than stating phase rules as facts.
Research has reported cycle-related changes in energy intake, but Wren does not apply estimated phase to calorie or macro targets during beta.
Research consistently shows that individual responses vary more than phase averages suggest. Wren does not present a calendar estimate as confirmation of ovulation or physiological state.
Period start and end, spotting, symptoms, and cycle settings are stored in Wren's app storage on your device, not on Wren's servers. Reinstalling or clearing app storage removes that history. Device or operating-system backups may include app data.
During beta, Wren records only whether cycle tracking is enabled and one later-return milestone. It does not send per-log events, symptom types, bleeding, spotting, period dates, cycle day, or estimated phase to analytics.
Full explanation: How Wren stores cycle data on your device. On the Wren Inside page.
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