Tracking your cycle: a guide, not a rule

The principle

Cycle tracking is optional: it is offered in the settings when your profile says you are a woman, and nothing appears until you turn it on. Once it is on, Azetta places each day in your cycle, predicts your next period and, if you want, lets you log how you feel today.

One idea guides everything else: the phases are a guide, how you feel today is the rule. On average, research finds very small differences in performance from one phase to another, and differences that vary a lot from one woman to another. So the app never changes a session because of the phase: it only suggests a gentler version if you, that day, logged low energy or pain. And it only suggests it.

The Azetta method

1. Your starting lengths

When you turn it on, you give the first day of your last period, your usual cycle length (21 to 40 days, 28 if you don't know) and your usual period length (2 to 10 days, 5 by default).

After that, your own logged periods are what count. The cycle length becomes the average of the gaps between the starts of your periods over your last 6 logged cycles, rounded to the day; the period length, the average of the periods whose end you logged. A gap of less than 15 days or more than 60 days is not taken as a cycle (a period forgotten in between, a typing error): it stays out of the average. The app tells you where the length it uses comes from: "based on the length you entered" or "average of your last logged cycles".

2. The four phases

Day 1 is the first day of your period. Ovulation is estimated 14 days before the next period:

• Period: the days of your period (logged, or the average length as long as the end is not logged).
• Ovulation (estimated): a 3-day window that ends on the estimated ovulation day.
• Luteal phase: the last 14 days of the cycle; its second half (the last week) counts as the late luteal phase.
• Follicular phase: the days between the end of the period and the ovulation window.

When the cycle gets longer, it is the follicular phase that gets longer, not the luteal phase: the estimated ovulation moves with your cycle length instead of staying on day 14.

3. Your next period and the calendar

The next period is expected at the start of your last period plus your average cycle length. The app shows the countdown ("next period in 6 days"), then, if it is late, the number of days it is overdue.

The calendar marks logged period days, expected ones and the days you logged how you felt. A cycle already followed by another keeps its real length; the cycles to come use the average. When a period is late, nothing is guessed beyond the expected date: predictions start again from the day you log it.

4. How you feel today

Everything is optional, and everything stays on your phone:

• Energy, from 1 (drained) to 5 (full of energy);
• Cramps or pain: none, light, moderate, strong;
• Sleep, from 1 (bad night) to 5 (great night);
• Mood, from 1 (very low) to 5 (great);
• a free note.

5. When a gentler session is suggested

The app only looks at what you logged today, in this order:

The phase alone never triggers anything: with no check-in logged, or with a check-in that looks fine, your session stays as planned, whatever the day of the cycle. When a suggestion appears, you choose: the lighter version (one set less per exercise), stretching instead, or keep the planned session. A run has no lighter version: stretching or the planned session.

6. Plate and hydration

A note for the day goes with the phase, without ever touching your calorie targets:

• during your period: iron-rich foods (pulses, red meat, fish, leafy greens), with a little vitamin C;
• follicular phase and ovulation: nothing special to change;
• luteal phase: appetite may rise, so it is better to plan filling snacks (protein, fibre) than to go without;
• last week before your period: drink a little more, and add a little more salt if you sweat a lot or it is hot.

7. Your data stays yours

Cycle data is stored on your phone. It is never part of your account's ordinary backup and is never shared with a coach you open your program to.

To keep it when you change phones, you can turn on an end-to-end encrypted backup, with a cycle password of your own (at least 8 characters; pick one that differs from your account password):

• the key is derived from your password and a random 16-byte salt by Argon2id (19 MiB of memory, 2 passes, 1 lane);
• your data is encrypted on the phone with XChaCha20-Poly1305, with a random 24-byte nonce for each backup; the header (format, Argon2 settings, salt) is authenticated too: change a single byte and opening fails;
• neither the password nor the key leaves the phone; the server only keeps an unreadable block.

Nobody can reset this password, not even Azetta: forget it and this copy is lost. One button deletes everything, on the phone and on your account.

Why this choice

• How you feel before the calendar. A systematic review with meta-analysis (McNulty et al., 2020) concludes that performance might be very slightly lower in the early follicular phase, but that this effect is trivial and varies a lot from one study to another, and recommends a personal approach rather than general rules by phase. That is why the app suggests nothing from the phase alone.
• The luteal phase anchored to the next period. Across hundreds of thousands of real cycles, it is mostly the follicular phase that varies from one woman and one cycle to another (Bull et al., 2019): estimating ovulation from the end of the cycle fits reality better than a fixed day 14.
• Your averages rather than a textbook cycle. Few cycles last exactly 28 days: the average of your last logged cycles predicts your next period better than a ready-made value.
• Argon2id with the OWASP settings makes each password guess costly in memory and time, which slows down brute-force attacks (RFC 9106). XChaCha20-Poly1305 encrypts and authenticates at the same time (RFC 8439 for ChaCha20-Poly1305), and its 24-byte nonce can be drawn at random for each backup with no risk of repetition.

Limits

• The phases are estimated from lengths, without any measurement (no temperature, no hormone test): this is neither a contraceptive method nor a medical device, and the app says nothing certain about your fertility.
• The calculation assumes a natural cycle of 21 to 40 days. If your periods are irregular, absent or painful, talk to a doctor.
• The plate and hydration tips are general: they do not replace the advice of a health professional.
• With no check-in logged, the app suggests nothing: that is on purpose.

References

1. McNulty KL, Elliott-Sale KJ, Dolan E, Swinton PA, Ansdell P, Goodall S, Thomas K, Hicks KM. The effects of menstrual cycle phase on exercise performance in eumenorrheic women: a systematic review and meta-analysis. Sports Medicine, 2020;50(10):1813-1827.
2. Bull JR, Rowland SP, Scherwitzl EB, Scherwitzl R, Gemzell Danielsson K, Harper J. Real-world menstrual cycle characteristics of more than 600,000 menstrual cycles. npj Digital Medicine, 2019;2:83.
3. Biryukov A, Dinu D, Khovratovich D, Josefsson S. Argon2 Memory-Hard Function for Password Hashing and Proof-of-Work Applications. RFC 9106, IETF, 2021.
4. Nir Y, Langley A. ChaCha20 and Poly1305 for IETF Protocols. RFC 8439, IETF, 2018.
5. OWASP. Password Storage Cheat Sheet (recommended settings for Argon2id). OWASP Cheat Sheet Series.