What is Brain Fog?
Understanding What's Happening Inside Your Brain
"I know the word… I just can't think of it."
"I've walked into the kitchen and completely forgotten why."
"I used to juggle work, family and life without thinking. Now I lose my train of thought halfway through a conversation."
If you've experienced anything like this, you're not alone.
Brain fog is one of the most common cognitive symptoms of perimenopause and menopause. For many women, it can also be one of the most unsettling—not because it stops them living their lives, but because it makes them question themselves.
"Am I just tired?"
"Is this normal?"
"Could this be the beginning of dementia?"
For many years, doctors couldn't fully answer these questions. Brain fog was often blamed on stress, poor sleep or simply getting older.
Today, that picture has changed.
Over the last two decades, research has shown that the menopausal brain undergoes genuine biological changes. Scientists can now measure subtle differences in certain aspects of thinking, observe changes in how the brain produces and uses energy, and better understand the important role oestrogen plays in keeping brain networks working efficiently.
The reassuring news is that these changes are very different from dementia. Rather than showing a brain that is failing, the evidence points towards a brain adapting to one of the biggest hormonal transitions of adult life.
Is brain fog real?
One of the first questions researchers wanted to answer was whether women were genuinely experiencing measurable changes in thinking, or whether brain fog was simply the result of poor sleep, stress or anxiety.
Professor Pauline Maki, a leading neuropsychologist who has spent much of her career studying cognition during the menopause transition, helped answer that question.
Rather than simply asking women whether they felt forgetful, her team used formal cognitive tests to measure attention, verbal learning, memory, working memory and processing speed.
The findings were both reassuring and important.
Women did not show a general decline in intelligence or overall thinking ability. Vocabulary, reasoning and long-term knowledge remained remarkably stable.
Instead, the measurable differences were concentrated in a much smaller group of cognitive functions—particularly verbal learning, verbal memory, attention and processing speed. Even then, the changes were generally subtle. Most women still scored well within the normal range, but the differences were consistent enough to confirm that the cognitive changes women described were real.
That distinction matters.
Brain fog doesn't mean your brain is failing.
It means some of your brain's most demanding mental processes have temporarily become less efficient.
Brain fog isn't one symptom
The term brain fog makes it sound as though one thing has gone wrong.
In reality, it describes several different cognitive functions.
Working memory is your brain's temporary workspace. It's the mental Post-it note that allows you to hold information in your mind for a few seconds while you use it. It helps you remember why you walked upstairs, follow a recipe or keep track of a conversation while planning what to say next.
When working memory becomes less efficient, information can disappear before you've had time to use it.
Processing speed has nothing to do with intelligence. It simply describes how quickly your brain can receive information, make sense of it and respond.
Many women describe knowing the answer to a question but needing a few extra seconds for it to come to mind. The knowledge hasn't disappeared. Retrieving it simply takes a little longer.
Verbal memory is perhaps the most recognisable symptom of all.
You know exactly what you want to say. You can picture the object. You've used the word hundreds of times before. Yet the word itself seems just out of reach. Then, five minutes later, it suddenly pops into your head.
That isn't the same as losing a memory. It's a temporary difficulty retrieving information that is already stored.
A glimpse inside the brain
To understand why these particular abilities are affected, it helps to know that the brain doesn't have a single "memory centre." Different regions work together, each performing its own specialised role.
One of the most important is the prefrontal cortex, the area just behind your forehead. Think of it as the brain's executive office. It plans, organises, prioritises, filters distractions and keeps information active while you use it. Every time you multitask at work, follow a conversation or remember why you walked into a room, your prefrontal cortex is hard at work.
Working alongside it is the hippocampus, a small seahorse-shaped structure buried deep within the brain. Its job is to help form new memories and retrieve recently learned information, including names, words and facts.
These regions don't work in isolation. They communicate through billions of tiny connections called synapses, where one neurone passes information to another. Every one of those messages requires energy.
This is where oestrogen becomes surprisingly important.
Why does menopause affect these parts of the brain?
For many years, oestrogen was thought of primarily as a reproductive hormone.
Research led by neuroscientist Professor Roberta Brinton transformed that understanding by showing that oestrogen is also one of the brain's key metabolic hormones.
It helps brain cells absorb glucose from the bloodstream and supports the mitochondria—the tiny power stations inside every cell that convert glucose into ATP, the energy that fuels every thought, memory and nerve impulse.
Oestrogen also helps maintain healthy synapses, allowing neurones to communicate efficiently, and influences important neurotransmitters including acetylcholine, which supports learning and memory, and dopamine, which helps regulate attention, motivation and mental focus.
In other words, oestrogen doesn't make you intelligent. It helps create the conditions that allow your brain to perform at its best.
Why brain fog comes and goes
Unlike menopause itself, perimenopause is characterised by fluctuating hormone levels rather than a steady decline.
As oestrogen rises and falls, the brain is continually having to adjust.
The systems that depend on oestrogen don't suddenly stop working, but they can become temporarily less efficient. Because the prefrontal cortex and hippocampus are among the brain's most energy-hungry regions, even a small reduction in efficiency can be noticeable.
That helps explain why brain fog often comes and goes. Some days you feel like yourself. On others, finding words, concentrating, or juggling several tasks suddenly feels much harder. Importantly, the memories themselves haven't disappeared. The knowledge is still there. The challenge lies in retrieving and managing that information quickly.
Can scientists actually see these changes?
Yes.
While Pauline Maki demonstrated measurable changes in cognitive performance, neuroscientist Dr Lisa Mosconi used advanced brain imaging to investigate what was happening inside the brain itself.
Her research found that, during the menopause transition, some regions of the brain use glucose less efficiently than before. She also observed temporary changes in brain connectivity and structure, suggesting that the brain is actively adapting as hormone levels change rather than becoming damaged.
One particularly fascinating discovery is that the brain doesn't simply give up when glucose becomes harder to use.
It can begin using ketones—an alternative fuel produced mainly from fat—as another source of energy. This appears to be one of the brain's natural ways of adapting to falling oestrogen levels.
However, glucose remains the brain's preferred fuel, and ketones don't seem to replace it quite as efficiently. Researchers believe it takes time for the brain to increase its ability to use ketones effectively, which may help explain why cognitive symptoms are often most noticeable during the hormonal fluctuations of perimenopause before many women begin to feel mentally sharper again after menopause.
Scientists are still investigating exactly how important this metabolic shift is, but it is one of the most exciting areas of current menopause research.
Is brain fog the same as dementia?
This is perhaps the question women worry about most.
Fortunately, the answer is usually no.
Menopause-related brain fog mainly affects attention, processing speed and the ability to retrieve information quickly.
Dementia, on the other hand, involves progressive damage to brain networks that increasingly affects memory, reasoning and independence.
With brain fog, you usually realise you've forgotten something. The word often comes back. The reason you walked upstairs eventually returns. Your long-term memories remain intact.
That pattern is very different from the progressive memory loss seen in dementia.
Of course, persistent, severe or rapidly worsening memory problems should always be discussed with a healthcare professional, but for the vast majority of women, brain fog during perimenopause reflects a brain adapting—not a brain failing.
Looking at brain fog differently
Perhaps the most important lesson modern neuroscience has taught us is this:
Brain fog is real. It isn't laziness. It isn't a lack of intelligence. And it isn't simply "all in your head."
Thanks to the work of researchers such as Pauline Maki, Roberta Brinton and Lisa Mosconi, we now have a far clearer understanding of why so many women notice changes in memory, concentration and mental clarity during perimenopause.
The science is still evolving, but one message has become increasingly clear.
Your brain isn't losing its ability to think.
It's learning to function through one of the most significant hormonal transitions of adult life.
Learn More About Your Menopause Journey
The more you understand what's happening inside your body, the less frightening it can feel. If you'd like to learn more, these guides are a good place to continue:
The Role of Oestrogen in the Brain
How Perimenopause Differs from Menopause
What's Happening to My Hormones?
Reference
Maki PM, et al. (2010) Summary of the National Institute on Aging Workshop on Cognitive Aging and the Menopause Transition. Menopause. 2010;17(4):815–822.
Maki PM & Henderson VW (2016) Hormone therapy, dementia, and cognition: the Women's Health Initiative 10 years on. Climacteric. 2016.
Mosconi L, et al. (2021) Menopause impacts human brain structure, connectivity, energy metabolism, and amyloid-beta deposition. Scientific Reports. 2021;11:10867.