The menstrual cycle has numerous biological effects on women. New SRI research asks: Could a better understanding of these patterns improve healthcare for women around the globe?
The biological function of the menstrual cycle is to enable pregnancy. However, pregnancy is only one of the effects of the menstrual cycle on health. As researchers study the menstrual cycle, they are realizing just how profoundly this powerful biological rhythm impacts the health of 50% of all humans.
“To understand the importance of the menstrual cycle on health, we can look to our understanding of another type of biological cycle: circadian rhythms,” comments Fiona Baker, who directs SRI’s Center for Health Sciences and its Human Sleep Research Program. Our bodies, she observes, are defined by numerous 24-hour or daily rhythms. Daily patterns are measurable in body temperature, heart rate, sleep-wake activity, immune reactions, and much more. We know that factors like sensitivity to pain and physical strength are dependent on the time of day and that eating the exact same food at different times can have different effects on health.
Could a more precise understanding of the menstrual cycle impact how we approach women’s health? Recent research indicates that both everyday healthcare and new medical innovations have much to gain by focusing on rhythmic changes related to the menstrual cycle.
How the menstrual cycle impacts women’s health
Multiple hormones (i.e., molecular messengers traveling in the blood) shift as a result of menstrual patterns, including estrogen, progesterone, LH, FSH, and inhibin B. These hormones drive the menstrual cycle, but since they are released into the blood, they can act all over the body and not just within the reproductive system. As an example, the estrogen receptor is ubiquitous, implying that almost any cell in the human body is equipped to be sensitive (and therefore responsive) to estrogen variations.
“To understand the importance of the menstrual cycle on health, we can look to our understanding of another type of biological cycle: circadian rhythms.” — Fiona Baker
Body temperature also displays menstrual rhythms, with variations of 0.3-0.7°C across the cycle according to a review written by SRI’s Dr. Fiona Baker. Temperature is one of the most critical physicochemical characteristics of the body because temperature influences all molecular reactions. A temperature change therefore has the potential to impact metabolism above and beyond any variations in our activities and environment.
New research: How the menstrual cycle impacts the heart, temperature, and sleep
In one key paper, Baker and her co-authors discuss an NIH-sponsored study of menstrual patterns conducted at UC Irvine and SRI’s Menlo Park campus. In contrast with many previous studies that relied on laboratory data and infrequent sampling, this study relied on a remote medical device (the Oura ring) to track participants’ finger temperature, heart rate, and sleep continuously in real time.
The study aligned with what other research methodologies have also shown about heart rate and temperature: The menstrual cycle does indeed seem to impact both temperature and heart rate. Furthermore, the research demonstrated that the temperature oscillation over a menstrual cycle can be modeled with the same kind of wave model, called a cosinor, that is used to model a body’s temperature change over the course of a day. With this modeling, it is then possible to compare menstrual temperature rhythms between individuals and to determine how these rhythms can be used as health indicators.
The researchers were also curious to know if the menstrual cycle was affecting sleep. Sleep is typically sensitive to temperature, and menstrual hormones have receptors in the brain regions controlling sleep. Some studies, performed in a sleep laboratory setting, have previously indicated that during the second half of the menstrual cycle, called the luteal phase, there is a reduction of rapid eye movement (REM) sleep, also known as paradoxical sleep. Studies have also found specific changes in brain activity during sleep in this second half of the cycle, with an increase in specific bursts of brain activity (called sleep spindles) that are important in memory consolidation.
While this research from SRI and UC Irvine tracked robust changes in temperature and heart rate across the menstrual cycle, it indicated that neither the duration of sleep, nor the number and duration of awakenings, nor the time needed to fall asleep changed across the menstrual cycle. “In our paper,” notes Baker, “we hypothesize that biological mechanisms may exist that actively compensate or buffer the effect of the menstrual cycle on sleep, to ensure that critical activities like sleep are consistently regulated regardless of physiological changes during the menstrual cycle.”
However, she cautions, future researchers (as well as medical practitioners) should keep in mind that the effects of the menstrual cycle on sleep may not be the same for everyone. For example, other research has shown important differences between individuals, with some experiencing a menstrual pattern of poor sleep around the time of menses, especially if the individual also experiences mood swings and/or menstrual cramps.
Future research into the connections between menstrual cycles and health
Future research will clarify the effects of the menstrual cycle on sleep and other health and performance measures, as well as identify potential mechanisms for further advancing women’s health. “To this end, our lab has recently developed two specific research tools to facilitate these investigations of the menstrual cycle and its effects on health,” Baker points out. One illuminating paper shows how wearable consumer devices that detect temperature can be leveraged to both detect that a cycle was ovulatory and extract characteristics of the menstrual cycle that can be compared between cycles and between individuals. Another method paper explains how an algorithm developed at SRI might allow researchers and healthcare providers to compare cycles of different lengths, target interventions for specific menstrual windows (for example, starting a treatment), and visualize the variations of health parameters across the cycle.
Long ignored by science and tabooed by societies, menstrual cycles have great potential to inform our understanding of women’s health. Emerging research into the nuances of the menstrual cycle is building a path toward truly personalized menstrual medicine.
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