This is Your Brain on Hormones
This Radiolab episode explores how hormones — particularly estrogen, progesterone, and testosterone — physically and functionally change the brain over time, both across the 28-day menstrual cycle in females and the 24-hour circadian cycle in males. Neuroendocrinologist Emily Jacobs and graduate student Laura Pritchett conducted a pioneering self-study called '28andMe,' scanning Laura's brain daily throughout her menstrual cycle to capture dynamic hormonal effects on brain structure and connectivity. The findings reveal that hormone fluctuations are a universal organizing principle of the brain, not a female-specific liability.
Summary
The episode opens with host Molly Webster posing a question she has carried for years: does the brain physically change across the menstrual cycle? To answer this, she speaks with neuroendocrinologist Emily Jacobs at UC Santa Barbara, whose lab specializes in how hormones affect the brain. Emily explains that hormones are chemical messengers released by endocrine glands — ovaries, testes, adrenal glands, the thyroid — that travel through the bloodstream and reach virtually every cell in the body, including the brain. Crucially, Emily argues that most prior research failed to capture the dynamic, rhythmic nature of hormones because it relied on single snapshots of hormone levels rather than tracking individuals over time.
Inspired by Stanford researcher Russ Poldrack's longitudinal self-scanning project, Emily's graduate student Laura Pritchett volunteered to undergo daily brain scans, blood draws, and saliva samples for an entire menstrual cycle in a study called '28andMe.' Laura was motivated partly by frustration that neuroscience studies overwhelmingly used only male animals, citing cyclical hormones in females as too complicated — a justification she found scientifically incoherent. Laura underwent rigorous daily MRI sessions while performing cognitive tasks, including resting-state scans, food reward tasks, and even pornography viewing, all while tracking her mood. She was intentionally kept blind to where she was in her cycle to minimize bias.
The results were striking. Around ovulation, when estradiol peaked, Laura's brain showed a massive increase in functional connectivity — the degree to which different brain regions communicate with one another. Emily uses a Christmas tree light analogy: all the lights go on simultaneously, with every region syncing up. As progesterone rose and estrogen fell in the week before menstruation, connectivity dimmed. Additionally, using high-resolution scans of the hippocampus — the brain region central to learning and memory — the team observed that the hippocampus physically grew and shrank across the cycle on a rapid timescale. This morphological change was confirmed when Laura went on birth control that suppressed progesterone by 97%, which completely eliminated the hormone-dependent hippocampal changes.
The episode then pivots to explore whether men experience similar hormone-driven brain changes. Laura's then-boyfriend (now husband), Pavel Shaptarenko, participated in a companion study called '28andHe.' Unlike the monthly female cycle, testosterone in men follows a circadian rhythm — peaking in the morning and dropping 30–70% by evening. Pavel underwent both morning and nightly scans across 30 days. The findings mirrored Laura's: when hormones were high in the morning, the brain was more interconnected and slightly physically larger; in the evening, connectivity and size decreased. Notably, men also have progesterone and estrogen, just at different concentrations, and these too cycle daily.
Emily and Molly discuss the broader implications of this research. One key point is that testosterone, often associated with male physiology, is actually more capricious and socially responsive than estrogen — it spikes and dips with social stimuli like watching a sports team win or lose — undermining the stereotype that females are uniquely 'hormonal' or variable. Studies have shown females are not inherently more variable than males in this regard; in fact, the opposite may be true.
The episode also explores a study by researcher Tali Baram examining what happens when a severe stressor is introduced during the high-estrogen window of neuroplasticity. In mice, high estrogen combined with extreme stress produced a PTSD-like phenotype that was long-lasting. Conversely, during low-estrogen periods, animals appeared more resilient — almost armored — against stressors. Emily speculates this suggests a biological logic to the low-estrogen phases of the cycle that we tend to experience as unpleasant: they may serve a neuroprotective function.
The episode closes with reflections on what it means to understand these rhythms. Emily describes hormones as an organizing principle for the brain — like a heartbeat or a tidal rhythm that the whole body syncs to. She questions the notion of the brain as a uniquely powerful, autonomous organ, suggesting instead that all of us are 'big biological sacks of chemicals' whose behavior is heavily shaped by hormones. She expresses hope that continuous hormone monitoring technology — akin to glucose monitors — could soon allow individuals to track their hormone levels in real time, opening up entirely new windows of scientific and personal understanding.
About this episode
We don’t have hormones, hormones have us.
Key Insights
- Emily Jacobs argues that prior hormone-brain research was fundamentally flawed because it used single-snapshot measurements, missing the essential dynamic nature of the endocrine system — the body responds to hormonal change, not static levels.
- Laura Pritchett's '28andMe' study found that around ovulation, when estradiol peaks, the brain shows a massive increase in functional connectivity across most regions, and the hippocampus physically grows — a morphological, not just functional, change.
- When Laura went on birth control that suppressed progesterone by 97%, the hormone-dependent hippocampal changes were completely eliminated, providing causal confirmation that the observed brain changes were driven by hormonal cycling.
- Pavel Shaptarenko's '28andHe' companion study showed that male brains undergo the same pattern of connectivity increase and physical expansion on a 24-hour circadian scale tied to testosterone's daily rise and fall — demonstrating that hormonal brain changes are not female-specific.
- Emily Jacobs argues that testosterone, despite being associated with male physiology, is actually more capricious and socially reactive than estrogen — it can be suppressed simply by watching a sports team lose on TV — making males arguably more hormonally variable than females in real-world conditions.
- Research by Tali Baram suggests that the high-estrogen window of neuroplasticity, while opening the brain to learning, also makes it more vulnerable to trauma — severe stressors introduced during this window produced long-lasting PTSD-like phenotypes in mice.
- Emily Jacobs speculates that the low-estrogen phases of the menstrual cycle, often experienced as cognitively foggy or emotionally difficult, may actually serve a neuroprotective function by armoring the brain against stressor-induced damage.
- Emily Jacobs proposes that hormonal rhythms function as an organizing principle for the brain — analogous to a heartbeat or circadian scaffold — suggesting that the predictable rise and fall of hormones may help the brain maintain optimal function rather than representing noise or instability.
Topics
Transcript
Hey friends, Lulu here. I have been working in public radio for over 20 years. I feel so lucky to get to do this job. And this is a moment unlike any other. Nearly a year ago, Congress eliminated our federal funding and so many of you stepped up to support Radiolab at that time. Thank you. Thank you from the bottom of our hearts. Thank you. But the truth is, we just aren't out of the woods yet. There's no sign of federal funding coming back. And New York Public Radio, our home station, is currently facing an ongoing shortfall of nearly $3 million. And so I'm coming to you to ask for support. We see you. We make…
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