IQ and brain fog — how brain fog affects cognitive test performance

You're writing a message and halfway through the sentence you forget what you wanted to say. You read the same paragraph three times and nothing sticks. You feel like your brain is wrapped in cotton wool. That's brain fog — and science is beginning to understand how deeply it affects what we measure as "intelligence."

What is brain fog?

"Brain fog" is not an official medical diagnosis, but a symptomatic descriptor — it describes a constellation of subjective cognitive difficulties:

• Slowed thinking
• Difficulty concentrating and sustaining attention
• Short-term memory problems
• Difficulty multitasking and planning
• A feeling of being "disconnected" from your own mind

The largest study to date characterising brain fog — published in Frontiers in Human Neuroscience 2024, based on 25,796 participants — describes it as a subjective impairment of concentration associated with functional difficulties in activities of daily living. Brain fog was associated with cognitive scores 0.1 standard deviations lower — equivalent to approximately 1.5 IQ points in the general population.

How many IQ points can brain fog take away?

Here the data are striking.

In February 2024, researchers from Imperial College London published in the New England Journal of Medicine results from a study of over 112,000 participants in England. The findings were clear:

• People who recovered from COVID-19 in 4–12 weeks had a cognitive deficit equivalent to 3 IQ points compared to uninfected individuals
• People with long COVID (symptoms > 12 weeks) — 6 IQ points
• People whose disease required hospitalisation — 9 IQ points

Memory and planning tasks were particularly affected. For scale: 9 IQ points is the difference between the 50th and 37th percentile — a difference visible in test results.

Most common causes of brain fog

Brain fog is a symptom, not a disease. It has many causes, each affecting cognitive performance differently:

1. Sleep deprivation

Sleep is not a luxury — it's a biological requirement for the brain. During sleep, the glymphatic system clears the brain of metabolic waste products. Without this process, toxins accumulate and disrupt neuronal function.

Research shows that chronic sleep deficiencies are linked to reduced cognitive performance and early signs of cognitive decline. Even a single night without sleep reduces working memory test scores to a level comparable with mild alcohol intoxication.

2. Inflammation (including post-COVID)

Research has revealed significant attention deficits in post-COVID patients, with evidence of reduced performance in tasks involving interference resolution and selective and sustained attention. Inflammatory cytokines can cross the blood-brain barrier and directly impair neuronal function. This is the mechanism by which infections, autoimmune conditions, and even chronic stress (which raises cortisol and pro-inflammatory cytokines) can manifest as cognitive fog.

3. Thyroid disorders

Thyroid hormones regulate the metabolic rate of every cell — including neurons. Low thyroid function slows brain metabolism and directly causes cognitive impairment. Symptoms are usually reversible with hormone therapy, though some people may have persistent cognitive symptoms despite normalised TSH.

4. Nutritional deficiencies

The brain needs specific nutrients to produce neurotransmitters. Deficiencies in B12, D, iron and magnesium are particularly often linked to cognitive symptoms. B12 deficiency can develop silently for a long time, until neuronal deficits become apparent.

5. Chronic stress

Cortisol, the stress hormone, is neurotoxic at high, chronic levels. The hippocampus — the area critical for memory and learning — is particularly sensitive to its effects. Chronic stress literally reduces hippocampal volume.

6. Other causes

Fibromyalgia, migraines, chemotherapy ("chemo brain"), menopause, coeliac disease, anaemia, diabetes, and many medications can cause brain fog symptoms.

How does brain fog affect IQ test scores?

IQ tests measure cognitive abilities at a given moment. If that moment is during a brain fog episode — the result will be lower. Which IQ components are most sensitive?

Most sensitive to brain fog:

• Working memory (holding and manipulating information)
• Processing speed (how quickly you react and complete tasks)
• Sustained attention (maintaining focus throughout the test)

Less sensitive:

• Verbal reasoning (if vocabulary base is intact)
• Visual-spatial reasoning (less dependent on sustained attention)

This is a similar pattern to ADHD — and not coincidentally: some mechanisms underlying brain fog (inflammation, stress, sleep deprivation) overlap with the neurobiology of ADHD.

What to do before an IQ test if you have brain fog?

Several practical guidelines:

Conditions before the test:

• Sleep well at least two nights before the test (not just one)
• Eat a balanced meal 1–2 hours beforehand (hunger impairs processing speed)
• Avoid alcohol for 48 hours
• Limit caffeine to a moderate dose (excess causes jitter and worsens precision)
• Choose a quiet environment without distractions

If you have chronic brain fog:

• Determine if you have a known cause (thyroid, deficiencies, sleep apnea)
• Test at your best time of day (for many people this is mid-morning after coffee)
• Treat the result as indicative, not final

If brain fog is post-COVID:

• People with unresolved persistent COVID-19 symptoms performed worse on cognitive tests in both speed and accuracy, particularly on memory and planning tasks. Results can improve over time — worth retaking the test after several months of full recovery.

Is brain fog reversible?

In most cases — yes. This is one of the most important pieces of information, because brain fog can be frightening for those who experience it.

Reversibility depends on the cause:

Summary

• Brain fog is a state, not a trait — an IQ test score during brain fog is underestimated and doesn't reflect your potential
• The 2024 NEJM study found a 3–9 IQ point deficit after COVID-19 depending on severity — a clinically significant difference
• Main mechanisms: impaired sustained attention, working memory and processing speed
• Causes are varied — sleep deprivation, inflammation, thyroid disorders, nutritional deficiencies, stress
• Most forms of brain fog are reversible when the cause is addressed
• Before an important test, ensure adequate sleep, food and a quiet environment

If your IQ test result was surprisingly low and you have been feeling tired, unwell or overwhelmed recently — revisit the test when your conditions improve. The brain isn't measured fairly when it's in survival mode.

Sources

• Hampshire, A. et al. (2024). Cognition and Memory after Covid-19 in a Large Community Sample. NEJM. DOI: 10.1056/NEJMoa2311330
• Alim-Marvasti, A. et al. (2024). Subjective brain fog: a four-dimensional characterization in 25,796 participants. Frontiers in Human Neuroscience.
• Herrera, E. et al. (2023). Cognitive impairment in young adults with post COVID-19 syndrome. Scientific Reports.
• Younas, A. et al. (2025). The Impact of Sleep Deprivation on Brain Fog, Cognitive Decline. PMC.