top of page

Factors That Can Influence Your Menstrual Cycle

Changes in the normal routine can theoretically affect fertility or the menstrual cycle. However, the effect can be very different, for example a single day or an entire cycle. Because every woman reacts differently to changes, it is generally important to know and observe your cycle.

These influencing factors can disrupt your body:

  • Sleep-wake rhythm

  • The annual time change can be a factor

  • Time zones

  • Shift work

  • Stress

  • Stress resistance

  • Weight, BMI and fat percentage

  • Natural influences

  • Environmental influences

Sleep-wake rhythm

Many of our physiological processes work according to a fixed rhythm, a 24-hour cycle called the circadian rhythm, our ‘inner clock’ that synchronizes with the day-night rhythm. Many hormones are affected by this rhythm: for example, the greatest amount of prolactin is produced in the hours after bedtime, while testosterone reaches its maximum in the morning hours. Our internal clock affects our sleep, digestion, body temperature, heart and blood pressure. On the other hand, this rhythm is influenced by factors from our environment: meals, exercise and stress. Our internal clock is important and essential for basal body temperature.

Clinical studies show that basal body temperature can vary depending on how much or how long you slept (1). For example, if you sleep less than four hours, you are likely to have a slightly lower temperature than if you sleep more than seven hours. This temperature difference can be explained by biorhythms. After a certain amount of time, certain physiological processes in the body - even when you sleep - need to get going because they cannot rest for too long. After how many hours of sleep this is the case varies from person to person and depends on the individual's lifestyle.

You can measure at any time with Daysy, as long as you have had at least 1 hour of good quality sleep, measure immediately after waking up in the morning, before getting up and becoming active. As described, shorter or disturbed sleep can lead to temperature fluctuations that can cause extra red or yellow days.

These temperature fluctuations do not affect the accuracy of Daysy. Our general advice is to measure as soon as you have woken up from your longest sleep of the night.

The annual time change may be a factor

In Europe, the clock is set forward one hour on the last Sunday in March (summer time) and back one hour on the last Sunday in October (winter time). You may have noticed some temperature fluctuations during this period, probably the background is that your body needs to get used to the new conditions.

Time zones

As beautiful as travelling can be, it can often also be tiring and stressful. A direct effect can be the delay or, in extreme cases, even the absence of ovulation. Travelling through time zones is often perceived as particularly unpleasant because our internal clock is out of sync. We all know this as jet lag. If you take Daysy on a trip, you will have noticed that you have some temperature fluctuations in the first few days, which is quite normal. Normally, the rule of thumb is that you need about 1 day per time zone for your body to get used to it.

Shift work

Shift work is usually accompanied by physical stress. Normally, this stress manifests itself in disturbed or poorer sleep. A 2002 study showed that 50% of midwives who work shifts experience irregular cycles (2). Cyclical changes mainly affect the follicular phase, i.e. the phase before ovulation, which in most cases led to a prolongation of the follicular phase (3). Interestingly, midwives who always worked night shifts had a relatively shorter but constant cycle (less than 25 days). Midwives who worked mixed shifts over a long period of several years had the greatest variability in cycles (3). Longer cycles or outliers are usually offset by Daysy. It is important to measure regularly to give Daysy as many indications as possible to calculate your fertile and infertile days.


Many studies have shown that stress in its various forms can directly affect your cycle (4). The physiology behind stress is very complex and largely not yet well understood. What is certain is that the adrenal cortex in particular plays a major role. Stress (physical and psychological) often manifests itself in a shortened luteal phase, the second phase of the cycle. Normally, the luteal phase lasts about 14 to 16 days. If it is shorter than 10 days, it is called luteal phase insufficiency (CLI). What causes luteal phase insufficiency? Progesterone, which is jointly responsible for the rise in basal body temperature and increased blood supply to the uterus, shares a basic building block with adrenaline, which is formed in the adrenal cortex. Under stress, a lot of adrenaline is produced and adrenaline takes away this basic building block. As a result, little progesterone is produced and the luteal phase is shortened. A US study found that women with stressful jobs (high demands but little control) were more than twice as likely to have a shortened luteal phase than women with less stressful jobs (5).

Daily exercise, to the point of exhaustion and associated physical stress, can lead to a change in cycle. Cycles without ovulation are especially difficult when exercising intensively (6).

Stress resistance

Natural resistance to stress varies during the cycle and is lowest around ovulation and in the luteal phase. The background is that the body is preparing for a possible pregnancy and the immune system (which is directly related to stress) responds relatively moderately. This is the only way to ensure that an embryo composed of own and foreign cells (i.e. genetically foreign) is not rejected by the body (7). This circumstance also explains why women often get sick during the luteal phase.

Weight, BMI and fat percentage

Your body weight not only affects your general health, but also directly affects your cycle. The Body Mass Index (BMI) allows you to estimate your body fat. BMI is calculated by dividing body weight (in kilograms) by height (in metres) in a square. From a medical point of view, a normal BMI is between 20-25 kg/m2. Women with a normal BMI have moderately the most constant and fewest cycles without ovulation. Not body weight, but the percentage of body fat has a direct impact on your fertility and your cycle. Oestrogen in particular is stored in body fat and contributes to about a third of the total oestrogen balance.

Overweight: Women who are overweight (BMI 25-30) or obese (BMI +30) and have a high body fat percentage have relatively frequent cycles without ovulation. This is because oestrogen, which is moderately stored in excess fat, prevents ovulation from occurring.

Underweight: The opposite is true for underweight women (BMI <20). The low fat content in the body prevents estrogen from being stored, resulting in less estrogen being available. About 50% of underweight women have irregular menstrual cycles. If you suffer from severe underweight, it is very important for your overall health that you seek medical advice and help under certain circumstances.


All the above factors are events that can affect the calculation of fertile and infertile days. In our recent study, ‘The Performance of a Fertility Tracking Device’, we systematically analysed how Daysy deals with physiological changes in the individual menstrual cycle (e.g. age, BMI, cycle length, skipping readings, high vs. low average temperature, temperature steps) and how these directly affect the algorithm calculations.

A total of 107,020 cycles from 5,328 women were included in the study!


Natural influences

As you may already know, every woman normally ovulates once per cycle. After ovulation, the mature egg is fertile for a maximum of 18 hours. Sperm can move and fertilise in the woman's body for up to five days under optimal conditions (around ovulation). Together, this gives a fertility window totalling six days. The fertile window is the period of your cycle when you can become pregnant. Since your cycle is subject to normal fluctuations, Daysy calculates a few extra potentially fertile days so as not to miss the fertile window.

One of the aims of this study was to find out how many of the days calculated as infertile (green) fell in the individual fertile (yellow/red) window. To ensure that ovulation was not missed, the fertile window was extended to eight days. The following cycle scenarios were included in the analysis:

Daysy cyclus tracker wetenschappelijk onderzocht nauwkeurig

Overall, only 0.6% of the days shown were green, even though they were in the fertile window and thus should have been ‘red’ (potentially fertile) (see graph). However, 50% of these ‘false green days’ were five days before ovulation and thus would have had only a minimal chance of pregnancy during this period.

Daysy studie uitkomsten

Environmental influences

Besides natural factors, there are also influences that can be influenced by the environment, i.e. from outside. These factors include the number of days measured and the normal fluctuation range in measurements (known as temperature outliers).

  • Outliers in temperature

  • Temperature outliers can be caused by some or a combination of many factors:

  • Too short or disturbed sleep

  • Getting up very early when this is atypical for you

  • Shift work

  • Unusual alcohol consumption

  • Stress, psychological tension, excitement

  • Change of environment (travel, holidays, major change of climate)

The second aim of the study was to find out how temperature fluctuations affect the calculation of fertile and infertile days. To get an answer, Daysy was ‘’powered‘’ with predetermined temperature data (see figure a-d).

cyclus verloop goede hormonale balans

ovulatie en menstruatie hormonale balans

progesteron hormoon na de ovulatie

stress en invloed op je hormonen, menstruatiecyclus verstoring PCOS

The analysis showed that temperature fluctuations have a direct impact on Daysy's calculations. When these fluctuations between measured days are small (see figure a), the algorithm calculates more green (infertile) days (56%). When the fluctuations are very large (see figure d), it calculates fewer green days (43%) and more yellow days (17%). In this way, you can see how Daysy adapts to individual circumstances. On average, Daysy users have temperature fluctuations as shown in figure c.

Number of unmeasured days

Another factor involved in calculating the number of fertile and infertile days is the number of measured days. Of course, there are always days when you skip measuring. That's part of life. Therefore, it was important for us to study the influence of the number of days measured on the calculation of fertile and infertile days. For this purpose, the datasets were divided into groups that had each measured between 0-20%, 20-40%, 40-60% or 80-100% of all days.

53.1% of users (a total of 47,800 cycles) who used Daysy for 80-100% of their cycle recorded an average of 41% fertile (red) days and 42% infertile (green) days. The analysis shows that Daysy adapts to individual situations.

meting met Daysy overslaan


1) Hibi, M. et al. Effect of shortened sleep on energy expenditure, core body temperature and appetite: a human randomised crossover trial. Sci. Rep. 7, 39640 (2017).

2) Labyak, S., Lava, S., Turek, F., and Zee, P. Effects of shiftwork on sleep and menstrual function in nurses. Healthcare for Women International , 23(6–7):703–714, 2002.

3) Attarchi, M., Darkhi, H., Khodarahmian, M., Dolati, M., Kashanian, M., Ghaffari, M., Mirzamohammadi, E., and Mohammadi, S. Characteristics of menstrual cycle in shift workers. Global Journal of Health Sciences , 5(3):163–172, May 2013.

4) Ferin, M. Clinical review 105: Stress and the reproductive cycle. Journal of Clinical Endocrinology and Metabolism , 84(6):1768–1774, Jun 1999.

5) Hatch, M. C., Figa-Talamanca, I., and Salerno, S. Work stress and menstrual patterns among American and Italian nurses. Scandinavian Journal of Work, Environment and Health , 25(2):144–150, Apr 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., and 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, Oct 2001.

Authors: Niels van de Roemer, Andrea de Groot



bottom of page