Long COVID Brain Fog: What the Research Shows and How It’s Assessed

Brain fog after COVID-19 is real and measurable in research studies, but it is not something a single test can confirm. The CDC lists difficulty thinking or concentrating, sometimes called brain fog, among the symptoms of long COVID, and notes that there is no approved laboratory test for long COVID itself. Large studies have found small average drops in thinking performance after infection, larger in people with ongoing symptoms. This guide explains what those studies did and didn’t show, which ones used the tasks now sold as Creyos, and how post-COVID fog is usually assessed.

What counts as long COVID

The CDC describes long COVID as a chronic condition that occurs after SARS-CoV-2 infection and is present for at least 3 months, adding that “there is no approved laboratory test that can determine if your symptoms or conditions are due to Long COVID” (CDC). The 2024 National Academies definition also uses a minimum of 3 months and does not require a positive test from the original infection (NASEM).

Among the symptoms the CDC lists are difficulty thinking or concentrating, tiredness that interferes with daily life, symptoms that get worse after physical or mental effort (post-exertional malaise), sleep problems, and depression or anxiety. The CDC also acknowledges that some symptoms are hard to explain or manage (CDC signs and symptoms).

What post-COVID brain fog feels like

People describe losing words mid-sentence, reading without absorbing, needing far more effort to do routine mental work, and crashing after a demanding day. In a study of how people use the term online, long COVID was the most common cause people gave for their brain fog (McWhirter et al., 2023).

Fog may not hold steady. The CDC notes that long COVID symptoms can improve, worsen or continue over time, which is one reason a single test session can under- or overstate how things are going. A short diary of good and bad days, sleep, and activity helps a clinician see the pattern.

The effort piece matters. If thinking hard makes you worse the next day, that is worth telling any clinician, because it affects how assessments and return to work should be paced. For the general question of whether fog can be measured, see is there a test for brain fog?.

The largest study: small average changes, bigger with ongoing symptoms

In England, researchers at Imperial College London invited 800,000 adults to complete an online set of eight cognitive tasks; 112,964 finished. Compared with people who had not had confirmed COVID-19, those whose symptoms had resolved showed small deficits in overall cognition (about 0.23–0.24 standard deviations). People with persistent, unresolved symptoms showed larger deficits (about 0.42 standard deviations), with memory, reasoning and executive function tasks most affected. Earlier variants and hospitalization were linked to larger differences (Hampshire et al., 2024).

Two points are easy to miss. First, these were averages across large groups; many individuals were unaffected. Second, the task scores correlated only weakly with people’s own reports of poor memory and brain fog, the same gap between feeling and measurement seen in other conditions. This study did not use Creyos; it used a separate online battery.

The study that used the Creyos tasks

A Canadian online study tested 478 adults who reported a positive COVID-19 test, on average about 30 days earlier, using the Cambridge Brain Sciences tasks that are now marketed as Creyos. Compared with pre-pandemic norms, they scored lower on processing speed, reasoning, verbal and overall performance, but not on short-term memory, and differences appeared even in people who had not been hospitalized. Lower performance tracked physical symptoms and illness severity, not the cluster of mental-health questionnaire scores (Wild et al., 2022).

Context matters here: the paper discloses that its senior author is the company’s chief scientific officer and its first author consults for it. Participants were self-selected volunteers, testing came soon after infection rather than at the 3-month mark, and the design was cross-sectional, so it cannot show cause and effect.

The studies side by side

What the treatment research shows so far

No treatment has been shown to reliably reverse post-COVID brain fog. In a small randomized trial, a home light-therapy headset was compared with a sham device over eight weeks, with the Creyos composite score as the main outcome. The difference did not reach statistical significance (p = 0.088), fatigue and mobility outcomes favored the sham group, and every author was paid by the device maker (Lim et al., 2026).

The NIH-created RECOVER Initiative is running trials aimed at thinking and memory problems after COVID (RECOVER). Until results are in, care focuses on treating what can be treated: sleep problems, depression and anxiety, medication side effects, and other medical contributors.

Fatigue, sleep and mood: the overlap

Post-COVID fog rarely travels alone. Broken sleep lowers attention on its own (see how poor sleep affects thinking). Depression slows thinking and makes everything effortful (see cognitive symptoms of depression). Interestingly, in Wild’s study the mental-health cluster did not track test scores, which argues against assuming post-COVID fog is “just stress.” But depression and anxiety are common after a long illness and deserve treatment in their own right. If low mood brings thoughts of self-harm, call or text 988, or call 911 in an emergency.

Midlife hormonal change, new medications and thyroid or nutrient problems can add to the picture as well; menopause brain fog and medication brain fog cover those.

How post-COVID fog is assessed, and whom to see

• Primary care first. A physical exam and blood work look for other causes, such as anemia, thyroid problems, low B12 or blood sugar problems, and for complications of the infection.
• Neurology or a long COVID clinic for new neurological symptoms, worsening memory, or fog with severe fatigue and post-exertional crashes.
• A psychiatric provider when sleep, mood, anxiety, ADHD or medication effects may be part of it.
• Neuropsychological testing when work, disability or legal decisions depend on a formal evaluation; Creyos is not neuropsychological testing.

At Willow & Stone, Dr. Stacey Forbes, PMHNP-BC, focuses on the sleep, mood and medication side, and may use Creyos as one input. A full battery is about 40 minutes of focused effort, which matters if exertion worsens your symptoms; ask our team about pacing and whether you would take it at home or in the office. See our brain fog evaluation page, or request a consultation.

Common Questions

Sources & Further Reading

• CDC — About Long COVID
• CDC — Signs and Symptoms of Long COVID
• National Academies — A Long COVID Definition (2024)
• Hampshire A et al. Cognition and memory after Covid-19 in a large community sample. N Engl J Med 2024 (PubMed 38416429)
• Wild CJ et al. Disentangling the cognitive, physical, and mental health sequelae of COVID-19. Cell Rep Med 2022 (PubMed 36103880)
• Lim L et al. Photobiomodulation for brain fog in post-COVID-19 condition: pilot RCT. EClinicalMedicine 2026 (PubMed 41768981)
• NIH RECOVER Initiative

This article is general education, not a diagnosis or advice for your situation. Creyos results support, and never replace, a clinical evaluation; only a clinician who has evaluated you can diagnose a condition or recommend treatment. Do not start, stop, or change a medication without talking to the clinician who prescribes it.