Brain fog: what it is, where it comes from and what a brain scan does and does not measure

You read a paragraph three times and still do not know what it said. You walk into the kitchen and forget what for. The word you are looking for will not come, and all day there is a layer of cotton wool between you and your work. Brain fog is the name people give it themselves: it is not an illness and not a diagnosis, but a description of how your thinking feels. Where it comes from and what you can measure yourself is set out below. The BioProfile brain scan measures how much those symptoms trouble you, and gives you an indication of where you stand today in this area, as the first of the 11 health areas the full scan assesses. This is not a diagnosis. It is insight, to see where you stand today. And to help you make better choices in your food and your lifestyle. What this scan does not do: test your thinking ability or make a diagnosis. Only a doctor can do that last one.

What brain fog is

Brain fog is not a term from a textbook but a word from people themselves. In research into chronic fatigue syndrome, patients describe it as slow thinking, trouble focusing, confusion, forgetting things and a haze over their thinking. On tests, researchers mainly see in them slower and less efficient processing of information, less attention and a smaller working memory.⁠¹

Of 138 people between 14 and 29 with a condition of the circulation who filled in a questionnaire, 132 reported brain fog. Those 132 most often described their fog as “forgetful”, “foggy” and “trouble focusing, thinking and speaking”, and as triggers they named fatigue (91 per cent), too little sleep (90 per cent) and dehydration (86 per cent).⁠³ That short list is the heart of it: brain fog is an experience of your thinking, and that experience moves with your sleep, your stress and your body.

Where brain fog comes from

The first place to look is your night. A meta-analysis of seventy studies with 147 tests showed that short-term sleep loss hits simple attention hardest: the moments when you briefly take nothing in.⁠⁴ It does not have to be a night without sleep. People who slept six hours for fourteen nights performed at the end as if they had not slept for up to two nights, and felt barely sleepier.⁠⁵

The second is tension. The front part of your brain, which steers your working memory and your attention, is the most sensitive of all brain areas to stress: even mild stress you have no grip on can reduce those abilities quickly and sharply.⁠⁶

The third is the menopause. In a meta-analysis, women after the menopause scored worse at remembering words after a pause than women before or during it, and the chance of low mood was higher during and after the menopause.⁠⁷

The fourth is an infection you have had. Of 81,337 people who took an online cognitive test, the people who had had covid performed worse than those who had not, even when they no longer had symptoms.⁠⁸

The fifth is your mood. People who rate their own thinking poorly often do not score worse on a test: the link between symptoms and measured performance is small, and the symptoms in themselves go together with low feelings.⁠² That does not make the complaint less real. It says where you usually have to look.

What a brain scan measures

The brain scan asks four questions about the past two weeks: mental fog, holding your focus, everyday forgetfulness, and whether thinking takes effort. Each question is given points, the points are added up and converted into a focus score from 0 to 100, with the reading band it falls into. Higher is better.

The questions are modelled on questionnaires about experienced everyday cognition. The best known, the Cognitive Failures Questionnaire, has asked since 1982 about everyday slips in noticing, remembering and acting; the score is as stable as a character trait and matches what a partner says about you.⁠⁹ Our questions are worded by us, not a copy, so the score is not a validated clinical score. We call it an experienced score, and we read it alongside your sleep and your wellbeing rather than on its own.

What a brain scan does not measure

Experienced thinking is not measured thinking. There is no fair way to measure cognitive performance with a questionnaire; what self-reporting captures is how your thinking feels, and that goes together only weakly with how you score on a test.⁠² So the brain scan is not a cognitive test and not a screening for a condition of memory or attention, and it does not make a diagnosis. A sudden, clear change in your memory or your language, or fog that gets worse instead of fluctuating, belongs with a doctor and not in a score.

What you do with your score

A low score usually points to sleep, tension or mood, and that is useful news: those are things you can do something about, and you can measure the effect. A score today is your baseline. Repeat the scan in two weeks, over the same questions, and you no longer have a feeling about your head but two numbers.

If you want to change something in those two weeks, start with your nights. The sleep loss you do not feel is exactly the loss that costs you your attention.⁠⁵ What a sleep test measures about that is set out in why you wake up tired.