What is brain fog, really, and where does it come from?
Brain fog has multiple causes that each work differently, from temporary adenosine build-up due to sleep deprivation to persistent brain changes in long COVID. Which cause applies to you determines what can be done about it, so if symptoms are long-lasting it is worthwhile discussing them with a doctor.
Brain fog is not an official diagnosis but an umbrella term for a cluster of complaints: difficulty concentrating, memory problems, and the feeling that your thoughts will not get going smoothly. It affects people in very different situations, from sleep deprivation to prolonged illness following COVID-19.
One of the best-understood causes is sleep deprivation. The longer you stay awake, the more adenosine builds up in your brain. Adenosine is a substance that suppresses the wakefulness system and promotes the drive to sleep. That build-up directly disrupts concentration and memory. Once you sleep, the level drops again and the symptoms disappear. This is a clear, biological explanation for the 'cloudy' thinking that follows a bad night.
Mental effort can itself cause a temporary form of brain fog. In a small study with 16 participants, 90 minutes of intensive cognitive work led to noticeable mental fatigue: afterwards, participants performed considerably worse on a cycling test than they had after a restful period. This was not about muscle or heart problems, but purely about how heavily the brain experienced the effort. This suggests that cognitive fatigue has a real, measurable effect on how you function.
In long COVID, brain fog is more persistent and more profound. After a COVID-19 infection, some people experience concentration and memory problems for months. Structural and functional abnormalities are visible on brain scans. The causes under investigation include: the virus persisting somewhere in the body, a dysregulated immune response, and damage to small blood vessels in the brain. Which of these pathways is the most important has not yet been established. Possible blood markers for that damage are still being studied and are not yet clinically usable.
Kidney dialysis is also associated with brain fog and cognitive complaints. Both the filtering of blood via a machine and the underlying kidney disease play a role. In the short term, confusion and a foggy feeling can occur; in the longer term, there is a risk of accelerated cognitive decline.
All claims are based on the supplied abstracts (PMID 38829253, 19131473, 38373361, 40275017). The strength of the evidence ranges from a small crossover study (n=16) to observational research in long-COVID and dialysis patients. None of the associations has been established through large randomised trials.