Brain fog is not a medical diagnosis. But it is real, it is common, and it almost always has a cause. Below are nine causes worth checking. For each one you get the same three things: what it is, why it makes your thinking foggy, and how to tell whether it is the one affecting you.
1. Poor sleep
Poor sleep is the most common cause of brain fog, by a wide margin. While you sleep, your brain files what you learned that day and clears out waste. Cut that short, or break it into pieces, and the next day you get slower recall, worse concentration and a shorter fuse.
The hours on the clock matter less than people think. Seven hours broken five times is not seven hours of sleep. Snoring, a baby, a night shift roster, a hot bedroom and a phone on the pillow all do the same damage.
How to tell if this is you: your sleep changed at roughly the same time your thinking did.
One thing to rule out: sleep apnoea, where your breathing stops and restarts through the night. It is badly under-diagnosed, and foggy thinking is one of its main symptoms. The giveaway is feeling unrested after a full eight hours. If you snore heavily, or someone has noticed you stop breathing, ask your GP. This one needs a test, not a better pillow.
2. Long-running stress
Stress that never switches off causes brain fog directly. Your working memory, the part that holds a thought while you use it, has limited room. Worry occupies some of that room permanently. There is simply less left for whatever is in front of you.
This is measurable. Sustained stress affects memory and concentration in ways that show up on tests. It is not a character flaw and it is not "just stress" in the dismissive sense.
How to tell if this is you: the fog is worse on the days with more on them, and it lifts on holiday.
3. Alcohol
Alcohol causes brain fog well beyond the next morning. The reason is sleep. Alcohol helps you fall asleep and then wrecks the second half of the night, which is the part that does most of the mental housekeeping. So you get the fog without the hangover, and you do not connect the two.
This applies at levels most people consider moderate. A couple of glasses most nights is enough.
How to tell if this is you: take two weeks off and see. It is the cheapest test on this page and it usually answers itself by about day ten.
4. Blood sugar swings
Your brain runs on glucose, so when your blood sugar drops sharply your thinking goes with it. A breakfast of toast and a flat white spikes it, the spike triggers a correction, and the correction overshoots. The result is a slump two or three hours later.
How to tell if this is you: your fog runs to a timetable. Same time most days, usually mid-morning or mid-afternoon, and it improves after you eat. If your fog has a schedule, this is the first thing to suspect.
What to do: add protein and fat to breakfast and see whether the slump moves or disappears.
5. Low iron, low B12, thyroid and vitamin D
These are four separate causes, and all four are found with the same cheap blood test. They are grouped here for that reason. Each one is common, each one causes foggy thinking, and none of them can be worked out by reading. You need the number.
Low iron. Iron carries oxygen around your body, including to your brain. When it runs low you get tired and lose concentration, and this happens well before it becomes anaemia. That is why the useful test is ferritin, which measures your iron stores, rather than haemoglobin alone. Most likely in women who menstruate.
Low B12. B12 keeps your nerves and blood cells working. Short of it, you get fog, pins and needles in hands or feet, and low mood. Left long enough it causes damage that does not fully reverse. More likely if you eat little or no meat, if you are older, or if you take metformin or reflux medication long term.
Underactive thyroid. Your thyroid sets the pace of your metabolism. When it runs slow, everything slows, thinking included. It usually arrives with company: feeling cold, constipated, tired, and gaining weight without changing anything.
Low vitamin D. Common in New Zealand through winter, and more so the further south you live. The link to fog is weaker than the three above, but it is on the same blood form, so there is no reason not to check it.
How to tell if this is you: you cannot. That is the point. If your fog has lasted more than a few weeks, ask your GP for these four before you spend money on anything else.
6. Perimenopause
Brain fog is one of the most commonly reported symptoms of perimenopause, the years of hormonal change leading up to menopause. Oestrogen affects memory and verbal recall directly, and during perimenopause it does not simply decline, it swings. Those swings are what people describe as fog.
It is also one of the most commonly dismissed symptoms. Women in their forties are routinely told they are stressed or depressed.
How to tell if this is you: the fog arrived alongside other changes rather than on its own. Changed or unpredictable cycles, broken sleep, night sweats, mood shifts. It is the cluster that identifies it, not the fog by itself.
What to do: see a doctor who takes perimenopause seriously, because there is real treatment for it. A supplement is not that treatment. We have written separately about hormone disrupting chemicals, which is a different subject and should not be confused with this one.
7. Medication you are already taking
A medication you take every day may be the cause of your brain fog, and this is one of the most overlooked explanations on the list. Plenty of common medicines list drowsiness, poor concentration or memory problems among their side effects. People read that, start taking them, and then forget the connection months later when the fog is just how life feels now.
The usual ones are older antihistamines for hay fever, sleeping tablets, strong painkillers, some blood pressure medicines, and some antidepressant and anti-anxiety medication. Long-term reflux medication belongs on the list too, but for a different reason: it reduces how much B12 you absorb, so it causes fog indirectly, over years.
How to tell if this is you: line up when the fog started against when you began or increased anything. If they match, you have a strong lead.
What to do: do not stop anything because of a blog post, including this one. Some of these are genuinely unsafe to stop suddenly. Put everything you take in a bag, including supplements, and take it to a pharmacist. They will go through it with you for free, they are very good at this, and you do not need an appointment.
8. A virus you had months ago
Brain fog that starts after an infection and does not lift is a recognised pattern with a name: post-viral cognitive impairment. COVID made it widely known, but doctors described it long before that with glandular fever and other infections.
How to tell if this is you: you can point to the illness. You were sick, you recovered in the ordinary sense, and your thinking never came back to where it was.
What to do: raise it with a doctor rather than waiting it out. Months of waiting is the common mistake here.
9. Damp and mouldy housing, the one New Zealand misses
The building you live in can affect how you feel, and in New Zealand far more of us live in damp, mouldy homes than anyone realises. This cause rarely makes lists like this one. Here it should, and the numbers are the reason why.
How common it actually is
From the 2023 Census: 20.6% of New Zealanders lived in a damp dwelling, and 16.5% lived in a house with mould larger than an A4 sheet of paper. That is roughly 859,000 people in a damp home and 694,000 living with visible mould.
One in five. Our houses are older, colder and less well insulated than those in most countries we compare ourselves to, and many of us have simply got used to it: a bit of black on the bathroom ceiling, a wardrobe that smells.
What a mouldy house exposes you to
Three different things, which get muddled together constantly. They are not the same and the evidence for each is different.
- Spores and fragments. Bits of mould you breathe in. These mostly cause allergy and airway problems: blocked nose, cough, worse asthma. This part is very well established.
- Damp itself. Independent of mould, cold and damp homes are linked to worse respiratory health in New Zealand research.
- Mycotoxins. Compounds that some moulds produce. At high doses they are genuinely toxic and nobody argues about that. Food safety and farming treat them as a serious problem, and binders are standard practice in animal feed for exactly this reason.
How to tell if this is you: go away for a fortnight. If the fog lifts while you are away and returns within days of getting home, you have learned something no blood test was going to tell you.
What to do: dry the house out. Fix the leak, heat it properly, use the extractor fan, deal with the wardrobe. Damp housing is bad for you on grounds nobody disputes, so this removes a real health risk whether or not it turns out to be touching your thinking.
P.S. Our product beGone does not treat brain fog. It does not treat mould illness. But it helps you to get rid of toxins that might be contributing to your problems. If you are living in a damp house, the binder is secondary intervention. The house is main problem. Fix the house first. That is our advice whether or not you ever buy anything from us. Then go and sweat those toxins out.
When to stop reading and see a doctor
Some things do not belong on a supplement company's blog. Book an appointment, reasonably promptly, if your fog comes with any of the following:
- It is clearly and steadily getting worse rather than staying level
- Weight loss you did not intend
- Getting lost somewhere familiar, or badly losing words
- Numbness, weakness, or changes to your vision
- New headaches, or a change in a headache pattern you already had
- Low mood or loss of interest lasting more than two weeks
- It started after a head injury
None of that is a reason to panic. It is a reason to be looked at rather than to keep reading
What to do, in order
- Get bloods done. Ferritin, B12, thyroid, vitamin D. Cheap, fast, and it rules four causes in or out at once.
- Fix sleep for two weeks. Properly, alcohol included. Most people find at least some of their fog here.
- Take your medication to a pharmacist. Everything, supplements included. Ten minutes, free, no appointment.
- Look at your house. Behind furniture, in wardrobes, around window frames, under the sink. Then dry it, heat it and ventilate it.
- Notice the pattern. Worse at home than away? Worse in winter? Worse at a particular hour? Patterns are the most useful thing you can hand a GP.
- Then, if you want to, think about reducing everyday toxins exposure. That is the part we can help with.
Made because I needed it
I had a mould problem and could not buy a decent binder in New Zealand, so I put one together. Four binders and ginger, one sachet a day, every batch tested.
Trial pouch of 10 sachets, or a box of 20. Free shipping over $99 anywhere in New Zealand.
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This page is general information, not medical advice. beGone is a food supplement. It is not intended to diagnose, treat, cure or prevent any disease. If you are dealing with symptoms, talk to a healthcare professional, particularly if you take prescription medication or are pregnant or breastfeeding. Take beGone 1 to 2 hours apart from other medications and supplements.