Brain fog: why you cannot think straight, and what to do about it
Brain fog is a symptom, not a diagnosis. It is the everyday name for a cluster of experiences: slow thinking, poor concentration, a word on the tip of your tongue, walking into a room and forgetting why. It is real and measurable, but it is almost always a downstream signal of something else, most often poor sleep, chronic stress, blood-sugar swings, a hormonal shift such as perimenopause or an underactive thyroid, low iron or B12, dehydration, certain medications, depression or anxiety, or the after-effects of an infection like COVID. The route out is to work out which driver is yours, check a short list of bloods, and treat the cause rather than the fog.
Key facts
- Brain fog is a description, not a medical diagnosis. There is no single test for it; the job is to identify the underlying driver, which is frequently more than one at once.
- Sleep loss is the most common and most reversible cause. Meta-analysis shows sleep restriction hits attention and vigilance hardest, with measurable drops in working memory and processing speed.1
- A large UK study (REACT, around 112,000 people) found COVID was linked to a cognitive deficit equal to about a 3-point IQ drop after mild resolved illness, 6 points with long COVID, and 9 points after intensive care.2
- Iron deficiency without anaemia can cause fatigue and foggy thinking; a normal haemoglobin does not rule it out, so ferritin matters.5
- NICE recommends a focused first-line blood panel for unexplained tiredness, including full blood count, ferritin, thyroid function, glucose or HbA1c, and coeliac screening, before reaching for anything exotic.8
What brain fog actually is
Brain fog is not a clinical term and does not appear in diagnostic manuals. People use it for some mix of slowed thinking, poor concentration, mental fatigue, forgetfulness and a general cloudiness. Because it is subjective, it is easy to dismiss, by others and by yourself, yet when researchers run objective cognitive tests the deficits often show up for real. That is the central point: brain fog is a genuine signal that your brain is not getting what it needs to run cleanly, whether that is sleep, fuel, oxygen-carrying iron, stable hormones, or simply a nervous system not stuck in overdrive.
It helps to separate it from two things it is not. It is not dementia, which is progressive, worsens over years and erodes daily function, whereas brain fog fluctuates with sleep, stress and the time of day and lifts when the cause is addressed. And it is not, on its own, a sign of a tumour or stroke; sudden, severe or one-sided symptoms are a different matter, covered at the end. For the gradual, fluctuating fog most people mean, the task is detective work: find the driver.
The common drivers, and how to spot yours
Most foggy thinking traces back to one or more of the drivers below. The pattern of your symptoms, when they started, and what makes them better or worse usually points to the likeliest culprits.
Poor sleep
This is the first thing to interrogate, because it is the commonest cause and the most fixable. In a classic meta-analysis by Lim and Dinges, sleep deprivation impaired cognition across the board, with the largest effects on sustained attention and vigilance, and clear hits to working memory and processing speed.1 You do not need a sleepless night to feel it: several nights of short or broken sleep accumulate into a daytime fog. Untreated sleep apnoea, where breathing repeatedly pauses overnight, is a frequently missed driver; tell-tale signs are loud snoring, witnessed pauses in breathing, and waking unrefreshed despite enough hours in bed.
Stress and cortisol
Sustained stress keeps the brain in a state poorly suited to calm, flexible thinking. The common assumption is sky-high cortisol, but the evidence on stress hormones is genuinely mixed, with chronically stressed and burnt-out people sometimes showing a flattened or blunted cortisol rhythm rather than a high one. The honest framing is that a nervous system stuck in threat mode is not a clear-thinking one. Our piece on the wired-but-tired nervous system explains why you can be exhausted yet unable to switch off, which is fertile ground for fog.
Blood-sugar swings
Sharp rises and falls in blood glucose can leave concentration patchy, particularly the slump that follows a refined-carbohydrate meal. The strongest evidence here is in people with diabetes, where both high and low glucose impair attention and memory; in people without diabetes the link is more mechanistic and observational, but a wobbly, mid-afternoon, post-lunch fog that eases with steadier meals is a recognisable pattern worth testing by changing what you eat.
Perimenopause and thyroid
Hormones are a major and under-discussed driver in midlife women. Around 40 to 60 percent report cognitive symptoms during the menopause transition, and longitudinal data confirm small but real dips in verbal memory that are not explained by age alone.3 The reassuring part, set out by Maki and colleagues, is that for almost everyone these changes stay within the normal range and tend to settle after menopause; only a small minority show clinically significant impairment.4 An underactive thyroid (hypothyroidism) classically causes mental slowing and forgetfulness alongside fatigue, weight gain and cold intolerance; the evidence is strongest for overt hypothyroidism, and more mixed for the milder subclinical form.4
Anaemia, iron and B12
Iron carries oxygen, and low iron starves the brain of it. Crucially, you can be iron deficient without being anaemic, and still feel foggy and exhausted, so a normal haemoglobin is not the all-clear; ferritin (your iron stores) is the marker that matters. A systematic review in women of childbearing age links iron deficiency to worse cognition, mood and fatigue, and an early randomised trial in non-anaemic adolescent girls found that correcting low iron improved verbal learning and memory.5 Low vitamin B12 and folate are the other classic deficiency causes: B12 in particular can produce fog, low mood and nerve symptoms, and a normal full blood count does not exclude it.6 Our guides on ferritin and iron deficiency and on B12 and folate cover how to read these results properly.
Dehydration
An easy one to overlook. In controlled trials, losing just over one percent of body mass through mild dehydration degraded mood, concentration and attention, and increased the perceived difficulty of tasks, in both young men and young women.7 This is not a reason to force litres of water, but if your fog tracks with a day of too much coffee and too little water, it is worth correcting first.
Medications
Many common medicines blunt thinking. Sedating antihistamines, strong painkillers, some older antidepressants and bladder medicines, sleeping tablets and benzodiazepines, and drugs with anticholinergic effects can all dull cognition, especially in combination or in older adults. Do not stop anything on your own, but a medication review is a sensible early step. Long-term metformin and acid-suppressing PPIs are also worth knowing about, because they can lower B12 over time.6
Long COVID and other infections
Cognitive symptoms are among the most common features of long COVID, and the effect is measurable. In the REACT study led by Adam Hampshire at Imperial College London (around 112,000 people in England), COVID was linked to a cognitive deficit equal to roughly 3 IQ points even after mild resolved illness, 6 points with ongoing long COVID, and 9 points after intensive care, with the largest hits to memory, reasoning and executive function.2 This is observational, so it cannot prove causation at the individual level, and encouragingly those whose symptoms had resolved performed much like people who never had lasting symptoms, suggesting it is often not permanent.2
Depression and anxiety
If your low mood or anxiety is persistent, or you are struggling to cope or to keep yourself safe, please reach out. In the UK you can speak to your GP, call NHS 111 and select the mental health option for urgent support, or contact Samaritans free on 116 123 at any hour. In an emergency, or if life is at risk, call 999. These states are common and treatable, and effective help exists.
Depression and anxiety very often present as brain fog. Low mood slows thinking and saps concentration and memory, and anxiety floods working memory with worry, leaving little capacity for the task in front of you. This is not the fog being "all in your head" in a dismissive sense; it is a real cognitive effect of a treatable condition. If the fog comes with persistent low mood, loss of interest or pleasure, or relentless worry, that points toward mood or anxiety as the driver, and toward help that works. Our pieces on low mood versus depression and constant worry and GAD go deeper.
The tests worth asking for
For unexplained tiredness and fog, NICE Clinical Knowledge Summaries recommend a focused first-line panel rather than scattergun testing.8 Notably, ferritin is often under-requested in practice despite being one of the higher-yield tests, so it is worth asking for by name.8
| Test | What it looks for | Why it matters for fog |
|---|---|---|
| Full blood count (FBC) | Anaemia, infection, other blood disorders | Anaemia reduces oxygen delivery to the brain.8 |
| Ferritin | Iron stores | Catches iron deficiency before anaemia appears; frequently the answer.5 |
| Thyroid function (TSH, with T4) | Under- or overactive thyroid | Hypothyroidism causes mental slowing and forgetfulness.4 |
| Vitamin B12 and folate | Deficiency states | Both cause fog and mood change; FBC can look normal.6 |
| Glucose or HbA1c | Diabetes, blood-sugar control | High and low glucose both impair attention and memory.8 |
| U&Es, LFTs, calcium | Kidney, liver and bone chemistry | Standard screen for systemic causes of fatigue.8 |
| Coeliac serology | Coeliac disease | An under-recognised cause of fatigue and fog.8 |
Evidence strength, plainly. The sleep, iron, B12, thyroid and COVID findings are well supported by meta-analyses, randomised trials or large cohorts. The blood-sugar and cortisol stories in people without diabetes are more mechanistic and observational, so treat them as plausible contributors to investigate, not settled facts. Reading several results together against your symptoms beats chasing any single number, the same pattern-reading we use across our health library.
Evidence-based ways to lift it
Because brain fog is a symptom, the most effective move is to treat the driver you have identified, not the fog itself. The levers below are where the evidence is strongest, and most people benefit from starting with sleep.
- Fix sleep first. Regular sleep and wake times, morning daylight, and a genuine wind-down. If you snore loudly, wake unrefreshed, or your partner notices breathing pauses, ask about a sleep apnoea assessment. Sleep is the single highest-yield lever, given how strongly sleep loss degrades attention.1
- Steady your blood sugar. Build meals around protein, fibre and slower carbohydrates rather than refined sugar and white starch, which softens the post-meal slump.
- Correct deficiencies, guided by results. If ferritin, B12 or folate is low, replacing them under medical guidance can lift fog and fatigue.56 Do not megadose blind; test, then treat.
- Hydrate sensibly. Aim for steady fluid intake across the day rather than catching up at night; even mild dehydration dents concentration.7
- Move, but do not punish yourself. Regular activity supports mood, sleep and cognition. Where fatigue worsens markedly after exertion, which can happen in post-viral states, pace rather than push.
- Treat mood and anxiety as conditions. Talking therapy (NHS Talking Therapies in England) and, where appropriate, medication can clear the cognitive fog that comes with depression and anxiety.
- Review medications and stimulants. Ask a pharmacist or GP about anything sedating or anticholinergic, and be honest about alcohol, which fragments sleep and dulls next-day thinking.
Be wary of treating brain fog as a supplement problem to be stacked away. Most over-the-counter "focus" products have thin evidence, and piling them on top of an unaddressed cause rarely helps. If you do want to use anything, run it through our stack builder first, and if you are not sure where to begin, the getting-started guide walks through changing one thing at a time.
When to see a GP
Most brain fog is benign and traceable to the drivers above, but some patterns need prompter assessment. See a GP if your symptoms are persistent, are getting worse, or are interfering with work or daily life despite attention to sleep and the basics; if there are deficiency or thyroid symptoms; if you suspect a medication is to blame; or if low mood or anxiety is part of the picture.
Seek urgent help, not a routine appointment, for sudden confusion, sudden severe headache, weakness or numbness on one side, drooping of the face, slurred speech, or loss of vision. These can signal a stroke or another emergency. In the UK, call 999. Brain fog that comes on gradually and fluctuates is different from any symptom that appears suddenly.
- Can we check a first-line panel including ferritin by name, full blood count, thyroid function, B12, folate and HbA1c, to look for a treatable cause?
- My iron studies or B12 are in range but I have symptoms: should we look at ferritin level and active B12 rather than just the normal flag?
- Could any of my current medications be contributing, and can we do a medication review?
- If I am in midlife with menstrual changes, could perimenopause be involved, and what are my options?
- Could this be related to mood or anxiety, and am I eligible to self-refer to NHS Talking Therapies?
References
- Lim J, Dinges DF. A meta-analysis of the impact of short-term sleep deprivation on cognitive variables. Psychological Bulletin. 2010;136(3):375-389. PMC3290659.
- Hampshire A, Azor A, Atchison C, et al. Cognition and memory after Covid-19 in a large community sample. New England Journal of Medicine. 2024;390(9):806-818. nejm.org.
- Greendale GA, Karlamangla AS, et al. Cognition in the menopause transition (SWAN). Study of Women's Health Across the Nation. PMC11285668.
- Maki PM, Jaff NG. Brain fog in menopause: a health-care professional's guide for decision-making and counseling on cognition. Climacteric. 2022;25(6):570-578. tandfonline.com.
- Pratt JJ, Khan KS. Iron deficiency, cognition, mental health and fatigue in women of childbearing age: a systematic review. Journal of Nutritional Science. See also Bruner AB, et al. Randomised study of cognitive effects of iron supplementation in non-anaemic iron-deficient adolescent girls. Lancet. 1996;348(9033):992-996. cambridge.org.
- National Institute for Health and Care Excellence. Vitamin B12 deficiency in over 16s: diagnosis and management. NICE guideline NG239, 2024. nice.org.uk.
- Ganio MS, Armstrong LE, et al. Mild dehydration impairs cognitive performance and mood of men. British Journal of Nutrition. 2011;106(10):1535-1543. See also Armstrong LE, et al. Mild dehydration affects mood in healthy young women. Journal of Nutrition. 2012;142(2):382-388. PMID 21736786.
- National Institute for Health and Care Excellence. Tiredness/fatigue in adults. Clinical Knowledge Summaries. cks.nice.org.uk.
- NHS. Tiredness and fatigue: 10 reasons you feel tired. nhs.uk.
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This article is educational and does not constitute medical advice, diagnosis, or a treatment recommendation. Medication uses described as “off-label” are not licensed for that purpose in the UK and should only be considered under qualified clinical supervision. Always speak to your GP, pharmacist, or a registered specialist before starting, stopping, or changing any treatment. If you have severe or alarm symptoms - unintentional weight loss, blood in your stool, difficulty swallowing, persistent vomiting, a fever, or severe pain - seek urgent medical care.