What is brain fog?

Have you ever been so tired you couldn't think straight, or gotten stuck rereading the same sentence over and over? For people who live with brain fog, these experiences are part of everyday life.

But what exactly is brain fog? And is it a cause for concern?

Officially, "brain fog" isn't a medical term. "It's the colloquial jargon that patients now use to try to communicate to their doctor what's going on," Amy Arnsten, a neuroscientist at the Yale School of Medicine, told Live Science. 

The term is typically used to describe a constellation of persistent symptoms, including difficulty focusing, mild confusion, "fuzzy" or sluggish thoughts, forgetfulness and a general sense of fatigue. Most people have some passing familiarity with these sensations, but if these feelings become chronic they can affect a person's quality of life.

Related: Can minds persist when they are cut off from the world?

Just because brain fog lacks a strict clinical definition doesn't mean it isn't real. "I see [persistent] brain fog as a sign that something is amiss," Sabina Brennan, a neuroscientist and author of the book "Beating Brain Fog" (Orion Publishing Group, 2021), told Live Science in an email.

What causes brain fog?

Patients presenting with brain fog may have an underlying health condition. It could be caused by some mild structural or functional damage to an area of the brain known as the prefrontal cortex, which is involved in cognitive processes such as planning and decision making. This region, which covers part of the frontal lobe, "is the most recently evolved part of our brains," Arnsten said, "and it has different neurotransmission than some of our older, more traditional circuits, like in sensory cortexes." These delicate neural circuits may be particularly sensitive to inflammation caused by infection or head trauma.

But brain fog can accompany a whole host of conditions. Inflammatory diseases such as multiple sclerosis or lupus can induce chronic brain fog, as well as bacterial infections like Lyme disease. Patients with depression or anxiety may be familiar with scattered, muddled feelings associated with brain fog. High blood pressure, low blood sugar, head injuries and sleep deprivation can all produce the effect, as can the onset of menopause. Even some medical treatments can induce brain fog — for example, certain blood pressure medications, sleep aids and chemotherapy.

But perhaps the most high-profile cause of brain fog currently is long COVID. People with long COVID can experience lingering symptoms weeks, months or even years after their initial SARS-CoV-2 infection. One of the most common symptoms reported by these patients is chronic brain fog.

A 2022 study published in the journal Nature found that COVID-19 can cause abnormalities and up to 2% more rapid gray matter loss in certain regions of the brain, including the prefrontal cortex. This, in turn, can impact people's cognitive abilities. "We found that the infected participants showed a greater decline in their ability to perform complex tasks compared with non-infected participants," Gwenaëlle Douaud, first author of the study and a neuroscientist at the University of Oxford's Nuffield Department of Clinical Neurosciences told Live Science in an email.

Can you treat brain fog?

The good news is that, in most cases, brain fog is treatable. "The human brain is pretty resilient," Brennan said. 

Identifying the underlying cause is key to addressing brain fog. If mental fuzziness is due to stress or lack of sleep, then something as simple as a good night's rest can alleviate it (though this is, of course, easier said than done). In addition, lifestyle changes such as exercise, eating well and stimulating the mind with puzzles can help minimize some cognitive cloudiness, according to Brennan.

But if a person's brain fog doesn't go away with quality sleep, then it's a good idea to get it checked out. "When a person is concerned about memory, it is always better to err on the side of caution and arrange a visit to the doctor," Brennan said.

Brain-structure changes from chronic conditions like long COVID may heal themselves over time, but there's still not enough data about the disease to know how long that could take. An analysis of 70 long-COVID cases found that those who reported concentration and memory problems continued experiencing these symptoms a year after infection, "indicating long-lasting symptoms," according to a 2023 study published in the BMJ. Vaccinated individuals were less likely to experience these lingering symptoms.

Researchers like Arnsten and her colleagues are searching for ways to ameliorate brain fog. They've found a couple of promising existing drugs (originally developed to treat other conditions, such as ADHD and high blood pressure), but their work is still in the early phases. "This is an arena where science is just starting to learn so much," she said. "The interface of the immune system and the neural system is very complex, and very important."