What is brain fog, really, and where does it come from?

Brain fog is not an official diagnosis but an umbrella term for a cluster of complaints: difficulty concentrating, memory problems, the feeling of thinking slowly or hazily, and trouble with planning and organising. It is a recognisable phenomenon across a wide range of conditions and situations, not one specific disease.

The best-known contemporary cause is long COVID. After a COVID-19 infection, the virus itself (or remnants of it), a dysregulated immune system, and damage to small blood vessels in the brain can together produce persistent cognitive complaints. How long and how severely those complaints last varies greatly from person to person.

Another well-documented cause is sleep deprivation. As you stay awake longer, a fatigue substance called adenosine accumulates in the brain. This substance suppresses your wakefulness system and directly contributes to the concentration and memory problems that people recognise as brain fog. With prolonged sleep deprivation or chronic insomnia, those complaints become clearly noticeable.

Mental fatigue from sustained cognitive effort is a third mechanism. A randomised study found that ninety minutes of intensive thinking caused participants to perform noticeably worse afterwards on an exhaustion test on a bicycle. Not because the heart, lungs, or muscles fell short, but purely because the effort felt heavier. Time to exhaustion dropped by around 15 percent. This shows that mental fatigue also has physically measurable effects.

Serious physical illnesses can also cause brain fog. In people with chronic kidney disease who undergo dialysis, doctors see accelerated cognitive decline, confusion, and headaches, on top of what the disease itself already causes. The treatment itself appears to play a role in this, independent of other co-existing conditions. This illustrates that brain fog is sometimes a side effect of intensive medical treatments.