When thinking gets worse, the fear tends to jump to the serious end. That is understandable and it is usually not where the answer is. The common contributors to cognitive dulling are mundane, frequently multiple, and — this is the important part — mostly treatable.
None of this is a substitute for assessment. It is the list a good clinician works through, so you know what to expect and what to raise.
Sleep, and specifically sleep apnoea
The largest single item on this list, and the most commonly missed.
Chronic insufficient sleep impairs attention, processing speed and working memory measurably. Most people underestimate their own deficit, because the subjective sense of sleepiness adapts while the performance cost does not.
Sleep apnoea deserves special mention because it is common, treatable, and produces exactly this presentation. The suggestive features are snoring, witnessed pauses in breathing, waking unrefreshed regardless of duration, morning headache, and daytime sleepiness. It is not confined to people who are overweight or older, and it is substantially underdiagnosed in women, partly because the presentation is more often fatigue and fog than loud snoring.
If nobody has asked about your sleep in any detail, that is the first gap to close.
Medications
Very often the answer, and very often not looked for because the medication has been taken for years without incident.
Categories worth examining: sedating antihistamines, particularly taken nightly for sleep; anticholinergic medications, which include some for bladder problems, some antidepressants and some for nausea; benzodiazepines and sleep medications; some blood pressure medications; opioids; some anti-epileptics; and several psychiatric medications.
Two additional points. Combinations matter — three medications each mildly sedating add up. And the timing of a start is powerful evidence: if fog began within a couple of months of starting something, say so explicitly, because the connection is easy to miss when the medication is for something unrelated.
Thyroid, iron, B12, glucose
Blood tests, cheap, and each capable of producing significant cognitive symptoms.
Hypothyroidism classically presents with slowing, fatigue and fog and is easily treated. Iron deficiency, with or without anaemia, impairs concentration. Common in people who menstruate and frequently under-investigated. B12 and folate deficiency cause cognitive symptoms, sometimes without anaemia. Risk is higher with vegetarian and vegan diets, some medications including long-term acid suppression and metformin, and with age. Blood glucose, both undiagnosed diabetes and the swings around meals.
Alcohol
At quantities most people do not consider a problem. The cognitive effect is not confined to the hangover; regular evening drinking fragments sleep architecture in a way that produces next-day fog even without feeling hungover.
The useful exercise is not a lecture but an experiment: two or three weeks off, honestly, and see. It is one of the few interventions on this list you can test yourself, and the result is informative either way.
Mood and anxiety
Depression impairs concentration, processing speed and memory directly — this is a core feature, not an add-on, and it can be prominent enough that the cognitive complaint is what brings someone in.
Anxiety occupies working memory continuously. A mind running a background process does not have the capacity it used to.
Both are treatable, and both are commonly present alongside something else rather than instead of it.
The aftermath of infection
Persistent cognitive symptoms following viral illness are well documented and taken seriously. The pattern frequently includes fog, fatigue disproportionate to activity, and symptoms that worsen after exertion — including mental exertion.
If your fog began after an illness, that timing matters and should be stated plainly. Management is a real area of clinical practice, not something to be waved off.
Hormonal change
Cognitive symptoms during perimenopause are commonly reported — word-finding, fog, memory. Often attributed to age or stress, sometimes for years. Worth naming directly as a possibility rather than waiting for it to be raised.
The things that get skipped
Dehydration and skipped meals. Sounds trivial, is not, and is trivially testable. Chronic pain, which occupies attention continuously. Sustained high stress, which impairs cognition measurably and is not a character question. Undiagnosed hearing loss, which produces effortful listening that presents as fog and inattention.
How to use this list
Not to self-diagnose. Two better uses.
Prepare. Go through it and note what applies: every medication, your actual sleep, your actual alcohol intake, any illness in the year before it started, whether anyone has mentioned snoring. That is the information that makes a fifteen-minute appointment productive.
Check coverage. If you have been assessed for fog and nobody asked about sleep, nobody reviewed your medication list, and no blood tests were done, the common causes have not been excluded — they have been skipped. That is a reasonable thing to raise, and asking "could we check thyroid, iron and B12?" is a normal request.
The part worth remembering
Several of these are often present at once, each contributing a little. That is why the answer to "what is causing my fog" is sometimes not one thing but three small things, and why fixing one produces partial improvement that gets misread as failure.
Partial improvement from one change is a finding. It usually means keep going.