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My temperature fluctuates – why, and what can I do about this?

In this article, we’ll cover what causes temperature fluctuations, how to spot them, and how you can adjust your measuring routine to improve fluctuations depending on which measuring device you use. Temperature fluctuations don’t affect Natural Cycles’ effectiveness as birth control, but you may get more Red Days (NC° Birth Control) or Brown Days (NC° Plan Pregnancy), which may reduce your satisfaction with the app.

Click into each of the sections listed below to access the information you are looking for:

What causes temperature fluctuations?

Our body temperature fluctuates daily, some days more than others. However, if your temperatures fluctuate a lot and don’t follow the pattern of lower temperatures in the follicular phase and higher temperatures in the luteal phase, the NC° algorithm will struggle with detecting your ovulation.

Temperature fluctuations can be caused by a number of things related to your measuring routine, for example:

  • How tightly or loosely the device sits on your skin (for those who measure with a wearable device)
  • Changes in sleep schedule (for those who measure with a thermometer)
  • The placement of the thermometer in your mouth
  • Illness
  • Alcohol consumption
  • Some medications

You should exclude any temperatures that meet these criteria when they occur. 

 

How can I see if my temperatures are fluctuating?

There are a few different ways to monitor temperature fluctuations during your cycles.

In-app messages

If your temperatures fluctuate a lot, the app may send you a message to let you know. 

The app will indicate that you have fluctuating temperatures if it’s having difficulties finding your ovulation day due to significant temperature variations. 

Your temperature variation refers to the degree your temperature typically shifts within a single cycle phase. If the algorithm flags your temperatures as fluctuating, it means your average temperature variation is unstable (i.e., your temperatures are varying significantly from one day to the next within a cycle phase).

High temperature variation in your Cycle Insights

Under the Cycle Insights tab in your app, you can see if your temperature variations for each cycle phase are considered stable or unstable (by clicking on Follicular phase and Luteal phase). If it shows "unstable," this means that your temperatures are fluctuating.

Keeping an eye on your graph

We recommend looking at your graph frequently—preferably after each temperature—to help you understand what impacts your temperatures and how to reduce fluctuations. A single unusual temperature doesn’t mean that your temperatures are generally fluctuating, but if you notice it happening often, you can consider whether there’s something in your routine that could be causing it.

In the graph below, you’ll find an example of what temperature fluctuations throughout the different phases of a cycle can look like.

Looking at the data for this user, we can see that the temperatures on Cycle Days 4, 9, and 14 (follicular phase) rose above the cover line — this would be considered temperature fluctuations. 

On Cycle Days 21 and 25 (luteal phase), the temperature suddenly dropped below the cover line — this is considered a temperature fluctuation because this temperature is unusual for this phase of the cycle. 

 

What does good temperature variation look like?

While the average temperature in the follicular and luteal phases differs from person to person, your temperature follows a curve with lower temperatures in the follicular phase and higher temperatures in the luteal phase. Remember that it’s normal for the temperature to vary a bit from one day to the next, even if your temperatures are stable overall.

  • During your follicular phase (from the first period day to ovulation), your temperature generally stays below your cover line. 
  • In the luteal phase (from ovulation to the next period), your body releases progesterone, raising your temperature above the cover line. 
  • When the temperature has risen enough to confirm ovulation, the most likely ovulation day is calculated from the temperature curve. You can read more about ovulation detection in this article: How Natural Cycles detects ovulation. 

The graph above shows an example of a cycle with stable temperatures in each cycle phase – with lower temperatures during the follicular phase and higher temperatures in the luteal phase.

 

What can I do if my temperature fluctuates when using the NC° Band?

Fluctuating temperatures while wearing the NC° Band can happen if: 

  • The band is not positioned correctly. Please ensure that you wear the band with the sensor on the inner wrist if you are not already doing this. You may experience more stable temperatures on your inner wrist due to the increased blood flow in this area. Additionally, we recommend wearing the band on your non-dominant hand. That is because this hand tends to move less during the night and thus produces better data.
  • The band is too tight or loose on your wrist. Your band should fit snugly, allowing sufficient contact with the skin for the sensors to obtain an accurate temperature reading, but not so tight that it becomes uncomfortable to wear overnight. The band should not move when you move your arm. 
  • You change the wrist the band is worn on. To minimize fluctuations, always wear the band on the same wrist and in the same position. If you have recently changed the band’s placement for any reason, wear it on that same wrist for a few weeks to determine if there is an improvement in fluctuations.
  • You're feeling sick or hungover. In this case, you should exclude your temperature so that the algorithm does not consider these temperatures.
  • You’re taking medication or hormones that affect your cycle and/or temperatures. We discuss this further in the following article: Can I measure while taking medication? 
  • You tend to switch sleeping schedules from one day to the next without any specific pattern. This may affect the quality of your data, and you can learn more about this in the following article: I work shifts, when should I sync data from my wearable device? 

 

What can I do if my temperature fluctuates when using the Oura Ring?

Fluctuating temperatures while wearing the Oura Ring can happen if: 

  • You wear the Oura Ring on a different finger each night. To limit fluctuations, you should always wear the ring on the same finger of the same hand. 
  • You're feeling sick or hungover. In this case, you should exclude your temperature so that it is not taken into account by the algorithm.
  • You are taking medication or hormones that affect your cycle and/or temperatures. We discuss this further in the article: Can I measure while taking medication?
  • You tend to switch sleeping schedules from one day to the next without any specific pattern. This may affect the quality of your data, and you can learn more about this in the article: I work shifts, when should I sync data from my wearable device?

Another possible reason for fluctuations is that your Oura Ring does not fit properly on the finger. If your ring feels too tight or loose on your finger, please refer to Oura's article, 'Find your perfect fit,' for guidance on the correct ring sizing. You can also reach out to the Oura team through this link so they can help ensure that you have the correct ring size on the correct finger.

Based on the data we’ve analyzed, we can share that fluctuating temperatures while wearing the Oura Ring are most often caused by rings that are too tight on the finger. Because hands tend to swell overnight, it’s a good idea to aim for a slightly looser fit.

If your Oura Ring turns out to be too tight (or loose), we suggest wearing it on a different finger (index, middle, or ring finger) on the same hand for at least a few weeks to see if this improves the fluctuations. We recommend choosing a finger where the ring is neither too tight nor too loose, so that it rotates smoothly on its own.

 

What can I do if my temperature fluctuates when using the Apple Watch?

Fluctuating temperatures while wearing the Apple Watch can happen if: 

  • The watch is too tight or loose on your wrist. Your watch should fit snugly, allowing sufficient contact with the skin for the sensors to obtain an accurate temperature reading, but not so tight that it becomes uncomfortable to wear overnight. 
  • You're feeling sick or hungover. In this case, you should exclude your temperature so that the algorithm does not consider these temperatures.
  • You’re taking medication or hormones that affect your cycle and/or temperatures. We discuss this further in the following article: Can I measure while taking medication? 
  • You tend to switch sleeping schedules from one day to the next without any specific pattern. This may affect the quality of your data, and you can learn more about this in the following article: I work shifts, when should I sync data from my wearable device? 

To help improve your data with Apple Watch: 

  • Try wearing the Apple Watch on your non-dominant hand. 
  • Try wearing a different band. From silicone to nylon, a range of band materials is available. We recommend you choose a band that is both comfortable to wear and helps your watch fit snugly while you sleep. 
  • Try wearing your Apple Watch on your inner wrist. While many people wear their watch on the outer wrist, sleeping with it on the inner wrist can provide more stable temperatures due to increased blood flow in this area. 

 

What can I do if my temperature fluctuates when measuring with a thermometer? 

If you are experiencing frequent temperature fluctuations while using a thermometer, your first step should be to review our measuring guidelines. Often, an incorrect thermometer placement in your mouth is the cause, and a simple, tiny adjustment to your routine can significantly improve the quality of your measurements.

If you've confirmed you are following our measuring guidelines, but your temperatures are still fluctuating, this can happen if you take your temperature when:

  • You have slept 2 hours more/less than usual (ex, on weekends)
  • You have already snoozed
  • Your partner, a child, or a pet woke you up close to your usual measuring time
  • You have slept very poorly
  • You feel hungover
  • You have already gotten out of bed before measuring
  • You feel sick
  • You take medication that affects your temperature

If any of the above apply to you, you should still measure your temperature in the morning (to stay in the routine), but you should exclude it when entering it into the app. 

Please also refer to the articles listed below, depending on what applies to you:

  • If you work shifts, please read more in this article: I work shifts, when should I measure with a basal thermometer? 
  • If you have a baby or small children who wake up at night, we recommend measuring after your longest stretch of sleep. Read more about this in the following article: My child wakes me up at night, when should I measure? 
  • If you travel across different time zones, exclude your temperature on the first 1-2 days to give your body time to adapt. After that, you can continue to measure as usual. Read more about this here: Can I measure while traveling?
  • If you often wake up at night, the article I wake up during the night; when should I measure? will help you identify the best time to measure.
  • If you’re unsure when to exclude your temperature, the article When and how should I exclude my temperature? may help. 
  • Do you have very few data points? If you've just started using Natural Cycles and think your temperature is fluctuating, the best thing to do is continue measuring until you complete one full cycle. A cycle begins on the first day of menstruation and ends when your next menstruation begins. Once you have a complete cycle's worth of data, the Graph View is the best place in the app to visualize your temperatures and easily spot any fluctuations.

Do you measure with a basal thermometer? To track your temperature with Natural Cycles, you should only use a basal thermometer (with two decimals) or one of our approved wearable devices (Which device is right for me?).

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