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Continuous Core Body Temperature, Circadian Rhythms, and Fertility

Continuous Core Body Temperature, Circadian Rhythms, and Fertility

Kindara | February 28, 2020 | Getting Pregnant
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Reviewed by: Dr. Ben Smarr, PhD, UCSD Bioengineering & Data Science at University of California San Diego

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For those who are trying to conceive, tracking basal body temperature (BBT), luteinizing hormone, and progesterone have historically been the recommended methods to track your fertility and/or predict ovulation, but now continuous core body temperature (CCBT) and circadian rhythm patterns are emerging as the gold standard for identifying when you are most fertile. Continuous core body temperature is a highly reliable marker for circadian rhythm patterns that correlate with the phases of the menstrual cycle and the release of related hormones, such as estrogen and progesterone. 

By monitoring CCBT and identifying the circadian rhythm patterns associated with ovulation, CCBT can predict ovulation earlier than traditional methods.  

Let’s get into the science of circadian rhythms and CCBT, and how they can help you take charge of your fertility journey.

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Your circadian rhythms — not the same as your biological clock 

Circadian rhythms are the physical, mental, and behavioral changes associated with the body’s 24-hour internal cycles. They’re closely correlated with almost all human functions, including body temperature, most hormone secretions, cognitive function, alertness, and sleep. 

Notice that we didn’t refer to circadian rhythms as internal “clocks,” though. That’s because biological clocks are not the same as circadian rhythms. Biological clocks are composed of specific proteins that interact inside cells in nearly every tissue and organ of the body. They produce and regulate circadian rhythms.

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Both biological clocks and circadian rhythms are naturally produced by the body but may also be affected by environmental factors — mainly daylight, which can turn on or off genes that control the structure of biological clocks. If the cycle of light and dark changes, circadian rhythms and biological clocks may speed up, slow down, or even be reset. 

In fact, circadian rhythms are the reason why jet lag and daylight saving time may make you feel tired and discombobulated, and why those who work the night shift may have difficulty sleeping during the day. 

Studies have shown that these circadian rhythm patterns can be identified by measuring core body temperature continuously. 


What is continuous core body temperature? 

Core body temperature is the body’s internal temperature, not the peripheral temperature that we’re used to measuring with an ear, mouth, or skin thermometer. Regardless of the temperature outside, your core temperature is tightly regulated to keep your body’s systems functioning properly. That’s why we sweat when we exercise and when the weather is hot — to help maintain a consistent core temperature. 

Peripheral body temperature measurements may be affected by a variety of factors. For example, heat is lost through the extremities; oral temperature may be affected by eating, drinking, and even smoking; and ear temperature may be affected by ear wax. Measuring core body temperature avoids many of the downfalls of these methods, and one study even showed that peripheral thermometer readings were as much as 1.8°F to 3.6°F (1°C to 2°C) lower than measurements of core body temperature. This difference may not seem like a big deal, but it is clinically significant and could change the treatment or diagnosis of certain conditions — imagine if an oral thermometer showed that your temperature was normal when you actually had a fever! 

Core body temperature is also a more accurate measurement when it comes to predicting fertility. When it is measured continuously, the patterns in temperature data illustrate dynamic changes correlated to circadian rhythms that have been shown to accurately pinpoint a woman’s fertile window.

How are CCBT and circadian rhythms related to my fertility and menstrual cycle?

The core body temperature pattern is influenced by the female reproductive hormones that govern the menstrual cycle. This temperature pattern is called the circamensal rhythm, which is layered on top of and affects the circadian rhythms. 

The circamensal rhythm variations create a subtle and recognizable temperature pattern in each cycle that appears prior to ovulation.

This pattern is only detectable by measuring CCBT, which makes CCBT a powerful tool for predicting ovulation earlier than other methods, and if you’re trying to get pregnant, having intercourse during these pre-ovulatory days significantly increases the probability of conception.

In addition to predicting ovulation, tracking CCBT may also detect other irregularities in circadian rhythms. If something is amiss with circadian rhythms, the chances are that the menstrual cycle is not functioning normally either. For example, abnormal cycle length may be associated with infertility, polycystic ovary syndrome, or luteal phase defect. Plus, disruption of circadian rhythms and sleep may be associated with infertility. Specifically, night shift work or jet lag have been associated with irregular, extended menstrual cycles, alterations in female hormone levels, an increased risk of pre-term birth, and overall reduced fertility.

Why can’t I just use basal body temperature to predict ovulation? 

It’s true, basal body temperature (BBT) has been successfully used for decades to both achieve and avoid pregnancy naturally. 

When measured daily during a menstrual cycle, the BBT pattern will generally be biphasic, meaning lower temperatures will be observed during the pre-ovulatory phase and then a dramatic shift to higher temperatures will be observed the day after ovulation and sustained until menstruation begins. Recording BBT is an inexpensive option to confirm ovulation after it occurs by identifying that temperature shift. Looking for a new way to identify your fertile window? Check out the Priya Personal Fertility System!


However, BBT has some drawbacks, especially for those who are trying to predict their fertile window to either avoid or achieve pregnancy. 

BBT is often referred to as your waking temperature, but it’s more accurately defined as your lowest temperature at rest, which generally occurs between 3:00 and 6:00 in the morning when most of us are still asleep. That means if you take your temperature when you wake up, it’s probably too late and doesn’t catch your true BBT.

Additionally, while BBT may confirm ovulation, it’s not great for predicting ovulation; therefore, BBT must be combined with another method (such as cervical mucus observations) for best results, regardless of your goal. 

On top of that, BBT requires vigilance and consistency: your temperature needs to be taken around the same time every day, before any other activity, and after sleeping for at least 3 consecutive hours. BBT may also be affected by illness, emotional or physical stress, alcohol consumption, medications, and a variety of other factors. In contrast, CCBT shows temperature changes due to female hormones and related circadian rhythms even when you’re in motion and not following a bunch of rules — in other words, when you’re living your normal life. 

Finally, BBT is not necessarily a guaranteed indicator of when or if ovulation occurred. That is, just because you don’t see a temperature shift doesn’t necessarily mean you didn’t ovulate, and even if you saw a temperature shift, that doesn’t mean ovulation happened precisely the day prior. One study found that in one-third of cases, the BBT shift was observed more than 24 hours after ovulation actually happened.

The case for CCBT

Compared to BBT and other ovulation prediction methods, CCBT does not require following certain rules and provides accurate and precise prediction of ovulation, regardless of what you’re doing. CCBT is also collected night and day to catch important changes in temperature patterns that could occur at any time. 

While BBT is an inexpensive, accessible tool that may provide some insight to your cycle and fertility… you’re probably not using a fax machine to send letters to colleagues anymore since you can email. So the real question is, why use BBT when you can use CCBT? 

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What about future applications of CCBT?

Measuring CCBT is an exciting new advancement in women’s health that has a wide range of potential applications, and fertility is just the tip of the iceberg. 

The menstrual cycle is considered a vital sign representing wellness, and with the data we can gather from CCBT, we could potentially learn even more about circadian rhythms of the menstrual cycle, pregnancy and labor, alertness and sleep, menopause, and more.  

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References +

Weschler, T. (2015). Taking Charge of Your Fertility: The definitive Guide to Natural Birth Control, Pregnancy Achievement, and Reproductive Health.

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