The Period Brain: How Hormones Change Behaviour - Dr Sarah Hill

The Menstrual Cycle Through an Evolutionary Lens

Dr. Hill begins by framing the menstrual cycle as a biological process optimized for human reproduction, which involves two fundamentally different tasks for females: attracting a high-quality mate and then supporting implantation and pregnancy. While much research has focused on the first half of the cycle when estrogen rises, triggering increased sexual desire, social engagement, and heightened ability to discern subtle cues in potential partners, the latter half of the cycle dominated by progesterone remains poorly understood.

She points out the evolutionary puzzle: why do so many women feel miserable during the last two weeks of their cycle—a period that constitutes almost half their reproductive years? From a natural selection perspective, it seems maladaptive for a large portion of the population to experience such distress regularly. Dr. Hill suggests that this discomfort relates to the deeply protective and preparatory role of progesterone as the body shifts focus from outward mating behaviours to creating a safe internal environment for pregnancy.

Hormonal Shifts and Brain Changes

Delving into neurobiology, Dr. Hill explains how estrogen in the follicular phase not only enhances energy and libido but also sharpens cognitive and sensory functions. For example, women near ovulation can distinguish fine differences in male testosterone markers, such as scent and facial features, which serve as proxies for genetic fitness and immune robustness. This hormonal surge equips women to select high-quality mates when conception is possible, with scientific studies confirming increased attractiveness, sexual motivation, and social engagement during this phase.

Once ovulation occurs, the empty follicle transforms into the corpus luteum, releasing progesterone, which modulates brain regions like the amygdala, lowering the threshold for perceiving threats. This heightened vigilance likely served ancestral purposes by keeping women cautious when they were physiologically vulnerable and preparing for the energy-intensive processes required for pregnancy. Symptoms commonly associated with premenstrual syndrome (PMS)—such as increased sensitivity, fatigue, food cravings, and mood swings—are shown to be a biological response rather than arbitrary dysfunction.

Progesterone's influence extends to physiological changes, including increased basal metabolic rate, appetite, and sleepiness, all aimed at conserving energy for potential gestation. The immune system shifts from a pro-inflammatory to an anti-inflammatory state to facilitate embryo implantation. Dr. Hill emphasizes that many women's struggles during this phase stem from a lack of understanding about these shifts and inadequate self-care practices that fail to accommodate their body's changing needs.

Female Sexuality Beyond Fertility

One of the most fascinating insights Dr. Hill offers pertains to why human females engage in sexual activity beyond fertile windows, unlike most mammals that restrict sex to estrus periods. Non-conceptive sex functions as a critical mechanism for pair bonding, facilitated by hormonal changes and neurochemical responses like oxytocin release. She highlights research showing that during the luteal phase, sex serves relational purposes, strengthening bonds and ensuring male investment, which historically was vital for fetal and infant survival.

This revelation flips the simplistic notion of female sexuality as purely driven by reproductive purpose and underscores the duality of motivations across the cycle. For example, in the second half of the cycle, women's sexual desire is more about emotional connection rather than pure physical urge or mating motivation seen in the ovulatory phase.

The Ovulatory Shift Hypothesis

The discussion acknowledges controversies around the ovulatory shift hypothesis—how clearly women's partner preferences change across their cycle. Dr. Hill notes mixed research findings but points to robust evidence that women's sexual motivation, attractiveness, and sensitivity to male cues reliably increase near ovulation. She clarifies that fluctuations in behavior are best understood through longitudinal studies tracking individuals over time rather than comparing groups, which often yields inconsistent results due to inter-individual hormone variability.

She also critiques the scientific community's historical neglect of female hormonal dynamics, where women were often excluded from research or studied only during low-hormone phases to maintain "male-like" consistency. Moreover, medical protocols and drug testing largely ignore cyclical changes, contributing to ineffective or harmful outcomes for women, such as inappropriate medication dosages or overlooked symptom patterns.

Impact of Hormonal Birth Control

Dr. Hill connects her earlier work on hormonal contraceptives with the physiological insights into the menstrual cycle, explaining how the pill suppresses natural hormonal fluctuations by maintaining steady levels of synthetic hormones. These synthetic "progesterones" differ significantly from endogenous progesterone in their brain effects and metabolism, leading to a reduction of beneficial neurosteroids that support mood regulation and mental health.

As a result, many women on the pill may experience decreased sexual desire, altered partner attraction, and increased risk of depression and anxiety—particularly in teenagers. Dr. Hill describes this hormonal flatline as a masking or loss of an authentic part of female neurobiology, which can impact satisfaction in relationships and overall well-being. At the same time, some women prefer the predictability and control the pill provides, highlighting the nuanced trade-offs involved.

Gender, Science, and Society

The conversation extends beyond biology into sociocultural critiques about how modern medicine, feminist discourse, and workplace expectations have marginalized or denied the significance of female biology. Dr. Hill condemns the pervasive, male-centered scientific paradigm that treats women as smaller versions of men, thereby disregarding sex-specific needs and experiences.

She stresses that acknowledging biological sex differences does not imply inequality or inferiority but rather is foundational for better healthcare, inclusive research, and social understanding. This recognition calls for a reframing of feminism to embrace difference without judgment and to challenge the "soft bigotry of male expectations" that assumes men's biology and behavior as the normative ideal.

Work environments, healthcare practices, and societal norms that ignore hormonal cycles may contribute to heightened burnout in women, as continuous, high-demand workloads clash with predictable physiological vulnerabilities during the luteal phase. Dr. Hill advocates for greater education and awareness so women can align their activities with their hormonal states, optimizing health and performance without requiring structural accommodation or stigma.

Future Directions

As a scientist and advocate, Dr. Hill encourages women to track and understand their own cycles personally, identifying how energy, mood, appetite, and social needs vary with hormonal changes. Better self-knowledge facilitates improved communication with partners and adaptation of lifestyle choices that enable women to "ride the wave" rather than feel overwhelmed by it.

She envisions that expanding research on the full menstrual cycle and its cognitive, emotional, and physiological impacts will empower women and men alike, creating a foundation for more equitable and effective healthcare and relationships. With rising attention to these dynamics, Dr. Hill hopes to see greater public and scientific acknowledgment of the menstrual cycle's importance as a core aspect of female identity and experience.

In closing, Dr. Hill underscores the need for transparency, education, and nuanced conversation about menstrual health and hormonal birth control, emphasizing that women deserve a comprehensive and accurate blueprint to navigate their biology confidently and holistically.