If you've noticed in your early forties that memory isn't quite as sharp, sleep has gotten rougher, or mood has taken turns that feel out of character — you're not imagining it, and it isn't stress. What you're experiencing has a biological explanation that researchers are only now articulating clearly.
Dr. Mary Claire Haver describes perimenopause as "its own very distinct biological transition — not a waiting room for menopause." It can begin as early as the late thirties and last anywhere from one to eight years. During that entire time, your brain is changing. This issue explains what those changes are, why they happen, and what you can do about them.
In brain imaging studies on healthy women aged 21 to 80, Dr. Lisa Mosconi measured how the brain processes glucose — its primary fuel — while scanning for Alzheimer's plaques, brain shrinkage, and vascular changes.
Post-menopausal women showed substantially lower metabolic activity — in some cases reduced by more than 30 percent. Crucially, similar reductions appeared in women still in perimenopause. Men of the same age showed minimal changes.
Your brain runs on estrogen — and perimenopause is its first fuel cut
Most of us learned that the brain runs on glucose. True — but estrogen is a key regulator of how efficiently that glucose is used. Dr. Mosconi frames estrogen as "fuel for the brain" rather than simply a reproductive hormone. When you see it that way, the magnitude of the perimenopause transition becomes much clearer.
Estrogen connects the hypothalamus — which governs hormone production — with the hippocampus (memory), the amygdala (emotion), and the frontal cortex (attention, language, reasoning). When estrogen begins to fluctuate in perimenopause, that communication gets noisy, and the brain has to find new ways to perform.
"Menopause doesn't 'cause' Alzheimer's. It's more like a trigger — in which the superpower of estrogen is revoked, and the brain has to find new ways to perform efficiently."
— Dr. Lisa Mosconi, The XX BrainFour brain symptoms you may already be experiencing
Brain fog and memory lapses. Up to 60 percent of women report reduced focus and mental clarity during perimenopause. These lapses are real and biologically grounded — not stress, not ageing prematurely.
Disrupted sleep. Dr. Stacy Sims explains that progesterone — the antianxiety hormone — has direct sedative effects and is a respiratory stimulant. As progesterone drops in perimenopause, sleep disturbances rise. Estrogen also increases REM sleep and assists serotonin metabolism. When both decline, sleep architecture falls apart well before hot flashes begin.
Mood instability and low-grade depression. Estrogen directly influences serotonin and dopamine pathways. Too often, women discuss perimenopause with their doctor and leave with antidepressants — when hormonal evaluation should come first.
Stress amplification. The "pregnenolone steal" — where cortisol production hijacks the same pathway that makes progesterone and estrogen — accelerates hot flashes, anxiety, and hormonal imbalance simultaneously. Chronic stress doesn't just feel worse during perimenopause; it biologically worsens it.
The Alzheimer's connection — and why timing is everything
Some women transitioning to menopause showed increased accumulation of amyloid plaques — a hallmark of Alzheimer's disease — along with metabolic decline and shrinkage of the brain's memory centres, despite showing no outward cognitive impairment yet. This is the earliest warning signal, and it opens a window.
Dr. Mosconi described Alzheimer's on The Diary of a CEO as being like a stock market crash — it doesn't arrive without warning, but appears at the end of a long sequence of changes, with the perimenopause transition as a critical inflection point.
The good news is that this is also the window of greatest opportunity. The earlier you understand what's happening, the earlier you can intervene with the lifestyle tools that move the needle most.
Dr. Haver described the moment she realised she was already in perimenopause — despite being a practising OB/GYN. She had been experiencing symptoms she didn't connect to the transition because, like most doctors trained before 2015, she had never been taught the full spectrum of perimenopause symptoms.
On The Diary of a CEO: "A lot of people misunderstand perimenopause as a waiting room for menopause. It really is its own very distinct biological transition — with its own set of symptoms."
Her advice: don't wait for a hot flash. Cognitive, emotional, and sleep changes often arrive first — and they're just as clinically valid as the physical ones.
The most useful wearable proxy for hormonal nervous system health is heart rate variability (HRV). A sustained HRV drop of 15–20% often precedes the subjective experience of cognitive fatigue and mood instability by several days — an early-warning signal worth logging alongside your symptom diary.
Establish your baseline this week. Check your HRV trend each morning for seven days and note any days of brain fog, poor sleep, or elevated stress. Patterns emerge quickly.
Dr. Mosconi's research shows that a Western diet high in refined sugars and processed fats accelerates brain ageing by the equivalent of up to five additional years. The Mediterranean diet — olive oil, oily fish, leafy greens, berries, legumes — is associated with significantly lower rates of cognitive decline and Alzheimer's plaques.
One swap, this week: Replace any refined seed oil (sunflower, vegetable, canola) with extra-virgin olive oil or avocado oil. Every meal. It's the single highest-leverage dietary change for reducing neuroinflammation, and it costs about 90 seconds to implement.
Dr. Mosconi's research shows that low-to-moderate intensity exercise — enough to elevate heart rate without preventing conversation — optimises metabolic performance in women around menopause. Five days a week of 30-minute moderate activity is associated with decreased risks of heart disease, diabetes, and cognitive decline. Higher intensity did not improve these benefits and in some cases reduced them.
This week: Two resistance training sessions + three 30-minute brisk walks. Both in your calendar, as appointments. This is the evidence-based minimum for this phase of life.