06/20/2025 1:11 p.m.

Influencing Factors That May Affect the Menstrual Cycle

Dr. Niels van de Roemer
Dr. Niels van de Roemer Medical Adviser
Influencing Factors on the Cycle

Understanding Factors That Influence Your Menstrual Cycle

Every woman’s menstrual cycle is unique and shaped by various internal and external factors. Elements such as sleep patterns, stress, body weight, environmental changes, and daily routines can impact cycle regularity and fertility. These factors affect hormonal balance, basal body temperature (BBT), and ovulation timing, making accurate cycle tracking essential.

The good news? Tools like Daysy and Lady-Comp are designed to adapt to these natural variations and still provide accurate fertility predictions.

No effect on the accuracy of Daysy

Your Internal Clock and Its Impact

Our bodies follow a 24-hour rhythm, known as the circadian rhythm or “internal clock,” which is linked to the Earth’s rotation. This rhythm influences essential functions such as sleep, digestion, body temperature, heart rate, blood pressure, and hormone production. For example, prolactin is mostly produced at night, while testosterone levels peak in the morning.

Environmental factors like meals, exercise, and stress can influence this internal rhythm, which in turn affects BBT, a key fertility indicator. Clinical studies show that BBT can vary depending on the quality and duration of sleep; sleeping less than four hours often results in slightly lower temperatures compared to a full night's sleep1.

Despite these variations, you can measure your temperature with Daysy as long as you’ve had at least one hour of good-quality sleep, taking the measurement immediately after waking, before getting out of bed. Short or disturbed sleep may cause minor fluctuations and a few additional “yellow” or “red” days (possibly fertile), but this does not affect the overall accuracy of Daysy.

No effect on the accuracy of Daysy.

no effect on the accuracy of Daysy

Impact of Annual Time Changes and Travel

Seasonal time changes, such as daylight saving time (when clocks move forward in March and back in October in Europe), can temporarily disrupt your natural rhythm. This may lead to slight temperature fluctuations or cycle irregularities while your body adjusts.

Traveling across multiple time zones (jet lag) can also disrupt your internal clock, sometimes delaying or even skipping ovulation for that cycle. Fertility trackers may show temperature outliers during the first few days of travel, but Daysy adjusts automatically. A general guideline is that your body needs approximately one day per time zone crossed to fully adapt.

No effect on the accuracy of Daysy.

no effect on the accuracy of Daysy

Shift Work and Irregular Cycles

Shift work, especially night shifts or alternating day/night shifts, can disrupt menstrual cycle regularity. A study from 2002 found that nearly 50% of midwives working shifts experienced cycle disturbances2.

Women working only night shifts tend to have shorter but consistent cycles (less than 25 days)2.

Those alternating between day and night shifts often have the most variable cycles3.

In such cases, consistent temperature tracking with Daysy is crucial for maintaining accuracy.

No effect on the accuracy of Daysy.

no effect on the accuracy of Daysy

Stress and Menstrual Health

Stress

Stress, both physical and emotional, can significantly impact menstrual cycles. The adrenal glands, which produce adrenaline and cortisol, share a metabolic pathway with progesterone, a hormone crucial for the luteal phase4. Under stress, your body prioritizes adrenaline production, lowering progesterone levels and sometimes shortening the luteal phase4.

Research shows that women in high-stress jobs are more likely to have luteal phase issues compared to those in lower-stress roles5. Intense exercise or extreme physical strain can also suppress ovulation, leading to anovulatory cycles (cycles without ovulation)6.

Stress Resistance and Immune Response

Stress tolerance varies across different phases of the menstrual cycle. It is generally lowest during ovulation and the luteal phase, as the immune system moderates itself to accommodate a potential pregnancy7.

This immunological adjustment is necessary to prevent the body from rejecting a fertilized embryo, which consists of both maternal and paternal genetic material7. Consequently, women may find themselves more susceptible to illness during these phases7.

No effect on the accuracy of Daysy.

no effect on the accuracy of Daysy

PMOS (Formerly PCOS) and Cycle Irregularity

Polyendocrine metabolic ovarian syndrome (PMOS, formerly PCOS) is one of the most common conditions affecting menstrual cycle regularity. At its core, most people with PMOS have insulin resistance: the body produces too much insulin, which triggers excess androgen production and disrupts ovulation. This can result in irregular, long, or anovulatory cycles, meaning cycles where no ovulation occurs at all.

Daysy can support you if your cycles fall within the 19 to 40 day range, including occasional cycles that exceed this range. In anovulatory cycles, your basal body temperature will not show the characteristic rise that confirms ovulation. Daysy will therefore show more red and yellow days until ovulation is detected or menstruation begins. This is the algorithm working correctly, not a limitation of the device.

If you have been diagnosed with PMOS (formerly PCOS), see our dedicated article: PCOS Is Now Called PMOS: What It Means for Your Cycle

PCOS Is Now Called PMOS: What It Means for Your Cycle
no effect on the accuracy of Daysy

Weight, BMI, and Body Fat Percentage

Body composition significantly influences hormonal balance and fertility. The Body Mass Index (BMI) is a common measure of weight relative to height, but body fat percentage plays a more direct role in menstrual regulation. Estrogen, a key reproductive hormone, is partly stored in fat tissue and contributes to approximately one-third of the total estrogen balance.

  • Overweight and Obesity (BMI 25-30 or higher): Excess body fat can result in elevated estrogen levels, leading to anovulatory cycles where ovulation does not occur.

  • Underweight (BMI below 20): Insufficient body fat reduces estrogen storage, which can cause irregular or absent menstruation. Up to 50% of severely underweight women experience menstrual disturbances, often requiring medical intervention.

Accurate Fertility Tracking Despite External Influences

All these factors—sleep, stress, weight, travel, or environmental changes—can affect the menstrual cycle and fertility window. However, Daysy’s algorithm adapts to these variations while maintaining accuracy.

What the Research Says

In the study “The Performance of a Fertility Tracking Device,” 107,020 cycles from 5,328 women were analyzed. Results showed:

  • Only 0.6% of days were incorrectly labeled as infertile (green) during the fertile window.

  • Of these, 50% were 5 days before ovulation, when pregnancy chances are very low.

The Study

Natural Fertile Window

Typically, a woman ovulates once per cycle. After ovulation, the egg remains fertile for up to 18 hours, while sperm can survive up to 5 days inside the female reproductive system. This creates a 6-day fertile window.

Because cycles naturally fluctuate, Daysy adds a few extra "possibly fertile" days to ensure the fertile window is not missed.

Environmental Influences and Temperature Outliers

External lifestyle changes can create temperature outliers (unusual readings), such as:

  • Short or disrupted sleep
  • Unusual waking times
  • Shift work
  • Alcohol consumption
  • Stress or emotional strain
  • Travel or climate changes

Temperature outliers and missed measurements

When temperature fluctuations are small, Daysy shows more green (non-fertile) days—around 56%. When fluctuations are high, green days drop to 43% and yellow (uncertain) days rise to 17%. This shows how Daysy adapts to individual circumstances. See example a-d.

Number of Missed Measurements

Skipping measurements is normal and accounted for by Daysy. The study found:

  • Women who recorded temperatures on 80–100% of days had an average of 41% fertile (red) days and 42% infertile (green) days.

This proves that Daysy remains accurate even with occasional missed measurements. See table below.

FAQ

Does short sleep affect Daysy's accuracy?

No. Lack of sleep can trigger extra yellow or red days, because basal body temperature tends to run lower on less sleep1. Daysy's accuracy in determining non-fertile days is not affected. Measure after at least one hour of restful sleep, immediately after waking.
Can I use Daysy with PMOS (formerly PCOS)?

es. Daysy supports you if your cycles fall within the 19 to 40 day range, even with occasional deviations. In anovulatory cycles, Daysy shows more red and yellow days until ovulation is detected or menstruation begins. This is the algorithm working correctly, not a limitation of the device.
Does stress affect the calculation?

Stress can shorten the luteal phase and change the cycle as a result4,5. Daysy adapts to these individual fluctuations and stays on red or yellow whenever in doubt, until the temperature rise is clear.
Does my weight change how Daysy evaluates my cycle?

Being over- or underweight can affect ovulation, because body fat is involved in estrogen balance. That affects your fertility itself, not Daysy's ability to detect the temperature rise. If ovulation doesn't occur, Daysy will correspondingly show more red and yellow days.

Sources

1)
Hibi, M. Effect of shortened sleep on energy expenditure, core body temperature and appetite: a human randomised crossover trial. Scientific Reports. 7 (39640). (2017).
2)
Labyak, S.; Lava, S.; Turek, F.; Zee, P. Effects of shiftwork on sleep and menstrual function in nurses. Healthcare for Women. 23(6–7), 703–714 (2002).
3)
Attarchi, M.; Darkhi, H.; Khodarahmian, M.; Dolati, M.; Kashanian, M.; Ghaffari, M.; Mirzamohammadi, E.; Mohammadi, S. Characteristics of menstrual cycle in shift workers. Global Journal of Health Sciences. 5(3), 163–172 (2013).
4)
Ferin, M. Clinical review 105: Stress and the reproductive cycle. ournal of Clinical Endocrinology and Metabolism. 84(6), 1768–1774 (1999).
5)
Hatch, M. C.; Figa-Talamanca, I.; Salerno, S. Work stress and menstrual patterns among American and Italian nurses. Scandinavian Journal of Work, Environment and Health. 25(2), 144–150 (1999).
6)
Reilly, T. The menstrual cycle and human performance: An overview. Biological Rhythm Research. 31(1). (2000).
7)
Pehlivanoglu, B.; Balkanci, Z. D.; Ridvanagaoglu, A. Y.; Durmazlar, N.; Ozturk, G.; Erbas, D.; Okur, H. Impact of stress, gender and menstrual cycle on immune system: Possible role of nitric oxide. Archives of Physiology and Biochemistry. 109(4), 383–387 (2001, Oct).

Learn more

Sleep-Wake Rhythm and Its Influence on Basal Temperature: How Your Circadian Rhythm and Basal Temperature Fluctuations Are Connected.

Sleep-Wake Rhythm

Learn how stress can affect your menstrual cycle and how a cycle tracker can help you detect early signs of stress. Get helpful tips for managing stress and improving your quality of life.

Stress & Cycle

Are you experiencing symptoms like missed periods, mood swings, or acne after stopping the pill? Get helpful tips and recommendations on how to rebalance your cycle naturally.

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