# The menstrual cycle, and the other context you record (BBT)

The things you tell VT about a day that are not measurements but change how the measurements are read: whether your period has started and where you are in your cycle, whether you had a bowel movement and what it was like, a note, and the stretches of time you mark as out of the ordinary.

Type: biomarker · Unit: cycle day; phase; °C; Bristol type 1–7; text; date range · Domain: Stress, Rest & Recovery · Accuracy: Self-reported · Importance: Supporting

Also called: period tracking, cycle phase, luteal phase, ovulation, basal body temperature, bowel movement, Bristol stool chart, daily notes, unusual period, holiday, annotations, why VT asks about my period

## In one sentence

The things you tell VT about a day that are not measurements but change how the measurements are read: whether your period has started and where you are in your cycle, whether you had a bowel movement and what it was like, a note, and the stretches of time you mark as out of the ordinary.

## Why it matters

The cycle is in every overnight reading. In 237 women who wore a sensor bracelet nightly for up to a year, wrist skin temperature, heart rate and breathing rate all shifted with the phase of the cycle, and heart rate variability and skin perfusion did too, with the luteal phase, after ovulation, running warmer and faster (Goodale 2019). In 22 women wearing a ring for an average of 115 days, nocturnal finger temperature was 0.30 °C higher in the luteal phase than in the follicular phase, against 0.23 °C for oral temperature on waking, and the ring's temperature alone located the start of a period within two to four days in 72 to 87 percent of cycles (Maijala 2019). A reading that is read against your own baseline without knowing your cycle day will call the luteal rise a change; with the cycle day, it is the cycle. The scale has the same problem on a different axis: the week before a period reads heavier and wetter, and a morning without a bowel movement reads heavier and higher in visceral fat because the current passes through the trunk (see [*Why VT asks about bowel movements*](https://vitalitytracker.app/knowledge/why-vt-asks-about-bowel-movements)). An unusual period, a holiday, a bereavement, a stretch of travel, explains a run of odd readings without pretending it is your new normal. A note is the one-line memory of the day that no sensor records.

## What you record

The cycle: each day's flow, from none to heavy, while you bleed; the first light or heavier day starts a cycle; VT then computes your cycle day and phase, six of them, menstrual, follicular, ovulation, early luteal, late luteal and premenstrual, anchored on the next expected period with ovulation placed 14 days before it, so the phases stay right for cycles that are not 28 days long; the expected date is learned from your own cycle lengths. Bowel movements: whether you had one that day, and for each, the time, its type on the Bristol chart from 1 to 7 and an optional note; the scale's morning question asks the first part. Unusual periods: when your readings shift, VT asks "Anything going on?", and your answer marks the stretch as out of the ordinary. Notes: free text for the day.

## How to record it well

Log each day's flow while you bleed; the first light or heavier day starts the cycle, and "no bleeding" ends the period. Nothing is needed for temperature if a ring is worn. Answer the bowel question before stepping on the scale, because it is read with that morning's weight. Answer VT's "Anything going on?" when it asks, and again when it checks back a week later; a sick day is a different mark, with its own entry, and the two are not interchangeable. Keep notes short; a line a day is what Eir can use.

## How Vitality Tracker uses it

On the days of your period, the overnight comparison gives resting heart rate, HRV, temperature and sleep efficiency less weight, and Eir is told your cycle phase when it explains a result; the energy engine treats the water retention of the cycle as a recurring level shift and subtracts it, personalized once enough cycles are recorded, rather than reading it as fat gained. Bowel movements feed the energy engine, which gives the morning's weight less weight from the second day in a row without one, and are kept with that morning's body composition reading. Unusual periods are used only to explain results afterwards; they never change your baseline, and a holiday that lifts your resting heart rate for a fortnight is shown as a holiday. Notes are shown on the day and read by Eir. VT does not estimate your fertile window, and the cycle phase it shows is context for your readings, not a basis for contraception or for timing conception.

## How far to trust the number

The cycle phase is calculated from your period dates and an assumed luteal length of 14 days, which is right for most cycles and wrong for some. The rest is self-reported, and its value is in being there: a missing bowel record is not read as no bowel movement, a missing note as nothing to say, and an unmarked holiday as a change.

## Method

Cycle day = days since the first day of the last period. Next expected period = last start + the average of your last six cycle lengths, leaving out any under 21 or over 40 days (28 days until two cycles are recorded). Ovulation = next expected period − 14 days; the six phases are bounded around those two dates as the engine specification sets them. Bristol types 1 to 7 as published. Unusual period = an annotation from the day your readings shifted, with the answer you chose, open until you say it has ended.

## Sources

1. Goodale BM, et al. Wearable sensors reveal menses-driven changes in physiology and enable prediction of the fertile window: observational study. J Med Internet Res 2019;21:e13404. [doi:10.2196/13404](https://doi.org/10.2196/13404)
2. Maijala A, et al. Nocturnal finger skin temperature in menstrual cycle tracking: ambulatory pilot study using a wearable Oura ring. BMC Womens Health 2019;19:150. [doi:10.1186/s12905-019-0844-9](https://doi.org/10.1186/s12905-019-0844-9)
3. Vitality Tracker, the engine catalogue of 15 October 2026 (the cycle, bowel, note and unusual-period records; the overnight comparison's confounders; the energy engine's cycle adjustment); Science hub, Methods (confounded days).

## Related

- [Why VT asks about bowel movements](https://vitalitytracker.app/knowledge/why-vt-asks-about-bowel-movements)
- [Body water and water weight](https://vitalitytracker.app/knowledge/body-water)
- [Overnight temperature deviation](https://vitalitytracker.app/knowledge/overnight-temperature-deviation)
- [Overnight resting heart rate](https://vitalitytracker.app/knowledge/overnight-resting-heart-rate)
- [Symptom Radar and Symptom Trend](https://vitalitytracker.app/knowledge/symptom-radar-and-symptom-trend)
- [The TDEE engine](https://vitalitytracker.app/knowledge/tdee-engine)
- [Sick days, holidays and unusual periods](https://vitalitytracker.app/knowledge/sick-days-holidays-and-unusual-periods)
- [What Eir can do, and what it won't](https://vitalitytracker.app/knowledge/what-eir-can-do)

Reviewed October 2026 · v1

Source: https://vitalitytracker.app/knowledge/menstrual-cycle-and-daily-context