Written By: Neta Gotlieb, Nina Thigpen, & Caroline Kryder

Your menstrual cycle naturally varies — understanding when and why it changes can be an empowering clue to your health.

What Changes?

Many elements can vary from cycle to cycle including the symptoms you experience, the timing of your ovulation or menstruation, and the heaviness of your period.

Your cycle is made up of key events within your follicular and luteal phases that change from person to person and cycle to cycle.

For example, one person might ovulate a few days before another or spend more time in their luteal phase than a friend. The experience for one person can even change from cycle to cycle; the same individual might ovulate on day 15 one cycle and day 18 the next. It’s even normal to have a cycle that skips ovulation from time to time, known as an “anovulatory cycle”. [1]

What’s behind all of these fluctuations? A symphony of hormones.

Why Do Cycles Vary?

Each event in your cycle is driven by the rise and fall of different hormones throughout your body, including estrogen, luteinizing hormone (LH), and progesterone. If even one of these hormone signals is “off” in its musical rhythm, it can disrupt your cycle or change the length of different events.

In the image below, you can see an example of a few of the hormones that have to coordinate their rhythms to keep cycle events in place.

How and when these hormones change are driven by your body’s response to circumstances in everyday life including: stress, sleep, travel, weight changes, age, nutrition, and underlying health issues.

This type of flexibility gives our body the ability to adapt from one day to the next and change the pattern of our hormones.

  • For example, if your body is under prolonged psychological or physical stress (think a major change at work, a severe flu, or unusually strenuous, prolonged exercise), it may stop ovulation by preventing the boost of luteinizing hormones known as your “LH surge”  that acts as your body’s “go signal” to ovulate. [2].

What does this mean for you?

The ovulatory cycle isn’t just about your ovaries. It can serve as a litmus test for your overall health — with major changes in stability or duration providing clues to pay extra attention to the interaction between your body, brain, and lifestyle.

These types of shifts can come as a surprise if we’re missing a method to spot these changes in our bodies and help us understand what happens next (e.g., menstruation or ovulation occuring on different days from one cycle to the next).

Keep in mind that many factors may impact your cycle variability: [3]

  • Cycles tend to grow shorter with age prior to perimenopause.
  • Cycles are impacted by your genetic background and body composition.
  • Cycles may show irregularity after recovering from past birth control usage, life stress, travel, an irregular schedule, illness, or over-training/prolonged athletic demands.
  • Cycles may become irregular due to a medical condition like obesity, metabolic syndrome or polycystic ovary syndrome (PCOS).
  • Cycles may change after giving birth. The return to cycling is strongly impacted by breastfeeding frequency, with more frequent feedings over a longer postpartum period preventing ovulation and menses for longer (don’t worry – this kind of cycle cessation is healthy for both you and your baby!).

What’s “Normal”?

Cycles are traditionally referred to as “regular” or “irregular” based on how cycle length varies. That length can change when any one of your cycle events becomes shorter or longer.

There’s a common misconception that most women’s cycles are 28 days long when, in fact, that may only represent ~13% of the population and cycles can range from ~20-40+ days. [4]  Moreover, many individuals have irregular cycles, often due to a variable length of time between menstruation and ovulation, which can make it hard to anticipate either fertile days or predict your next period.

How your body changes from cycle to cycle can be a key window into whether your body is fluctuating normally or if there’s something like stress, illness, or a medical condition that may need your attention.

How Oura Data Can Help

Currently, there aren’t many available tools that can help you learn about your personal cycling patterns.

  • Do your cycles tend to be similar?
  • Do they change around certain habits (e.g., diet), life events (e.g., work stress) or reproductive changes (e.g., birth control, returning to cycling postpartum)?
  • Do they change with age?

Researchers at Oura wanted to help answer these types of questions by compiling anonymized data across 40,000 menstrual cycles from the Oura Community. The results demonstrate that Oura’s temperature data can provide a personalized signal for individuals who want to learn more about their cycles. This research expands on existing findings that Oura can be used as a helpful tool (i.e., for menstrual cycle phases and key fertility signals).

Here are the top findings:

Oura Temperature Data May Help You Understand Clear Cycle Patterns

Temperature naturally fluctuates across the menstrual cycle because it is highly impacted by hormones like estrogen and progesterone. Although there are some complex dynamics at play, generally:

  • Estrogen decreases your temperature in the first half of your cycle
  • Progesterone, in combination with estrogen, raises your temperature in the second portion of your cycle, after ovulation.

Menstruation occurs at the start of each cycle, following a decline in estrogen, progesterone, and temperature.

While some cycle and fertility tracking measurements rely on oral thermometers estimating your “BBT” or basal body temperature once per day, Oura measures temperature 24/7, directly from your skin. [Read More]

Oura measures your nighttime body temperature relative to your personalized baseline, so you can easily see when your temperature starts to drop near menstruation or changes for each cycle phase.

Nocturnal Oura temperature deviation exhibits a biphasic pattern across the cycle [Learn more], and researchers have found that the continuous temperature signal gives users the tools so that they can interpret their data,  which an oral thermometer can’t do well, like the preovulatory LH surge [Learn More].

In the graph below, you can see an example of temperature rising and falling over three cycles for one individual. Using this distinct pattern, Oura researchers can investigate how cycles change over the course of many days or months.

Oura Data Can Identify Long, Short, And Irregular Cycles

While the initial example above follows a consistent pattern, we know that cycles have incredible variability. Users can review their temperature data and look for cycle patterns.  Users can review their temperature data and look for cycle patterns like the examples below that are long (58 days between periods), short (20-25 days), or highly variable.

Note: In the image below, the follicular phase, luteal phase, and ovulation are estimated.

Oura Data Captures Population Variation

Although the “average” cycle length was around 28 days, cycles varied in length from 20-40 days and each individual showed a personal variation of +/- 8 days from one cycle to the next on average. Note that in each case, menstruation was reported on during a time of declining temperatures during the late luteal phase.

What Does This Mean For You?

These results confirm established research showing that cycles vary widely and highlight the importance of understanding your personal cycle.

In your own data, you can look for these patterns in your Trend view and use Tags to monitor cycle events and symptoms:

Researchers at Oura will continue to look for meaningful patterns, partner with outside experts, and share what we learn with Oura members.

In the meantime, if you’ve seen changes in your own cycle that might empower others – share your story with us.

Read More:

Meet The Authors:


Note: All data was observed by Oura researchers as aggregated and anonymized, and not for any specific user in accordance with our privacy policies (link).


[1] Direito A, Bailly S, Mariani A & Ecochard R (2013). Relationships between the luteinizing hormone surge and other characteristics of the menstrual cycle in normally ovulating women. Fertil Steril 99, 279–285.e3.

[2] https://pubmed.ncbi.nlm.nih.gov/26393311/

[3] Bull, J. R., Rowland, S. P., Scherwitzl, E. B., Scherwitzl, R., Danielsson, K. G., & Harper, J. (2019). Real-world menstrual cycle characteristics of more than 600,000 menstrual cycles. NPJ digital medicine, 2(1), 1-8.

Gnoth, C., Frank-Herrmann, P., Schmoll, A., Godehardt, E., & Freundl, G. (2002). Cycle characteristics after discontinuation of oral contraceptives. Gynecological Endocrinology, 16(4), 307-317.

Bracken, M. B., Hellenbrand, K. G., & Holford, T. R. (1990). Conception delay after oral contraceptive use: the effect of estrogen dose. Fertility and sterility, 53(1), 21-27.

Nassaralla, C. L., Stanford, J. B., Daly, K. D., Schneider, M., Schliep, K. C., & Fehring, R. J. (2011). Characteristics of the menstrual cycle after discontinuation of oral contraceptives. Journal of Women’s Health, 20(2), 169-177.

Harlow, S. D., Campbell, B., Lin, X., & Raz, J. (1997). Ethnic differences in the length of the menstrual cycle during the postmenarcheal period. American journal of epidemiology, 146(7), 572-580.

Leeners, B., Geary, N., Tobler, P. N., & Asarian, L. (2017). Ovarian hormones and obesity. Human reproduction update, 23(3), 300-321.

Pauli, S. A., & Berga, S. L. (2010). Athletic amenorrhea: energy deficit or psychogenic challenge?. Annals of the New York Academy of Sciences, 1205, 33.

[4] Bull, J. R., Rowland, S. P., Scherwitzl, E. B., Scherwitzl, R., Danielsson, K. G., & Harper, J. (2019). Real-world menstrual cycle characteristics of more than 600,000 menstrual cycles. NPJ digital medicine, 2(1), 1-8.