What causes brain fog? Common causes and how to clear it
Brain fog isn’t a diagnosis. It’s a cluster of symptoms with real, often treatable causes. Here’s what the research says about where it comes from and what helps.
“Brain fog” describes a feeling most people know: thoughts move slowly, words won’t come, you reread the same paragraph three times and still lose the thread. Doctors don’t treat it as a diagnosis. Cleveland Clinic calls it a term for a range of symptoms affecting thinking, focus, concentration, memory and attention. The useful question is what’s causing it, because many of the causes can be fixed.
What brain fog feels like
- Trouble concentrating, or losing your train of thought mid-sentence
- Forgetfulness: walking into a room and not knowing why
- Struggling to find words you know well
- Slow thinking or slow reactions
- Mental exhaustion after tasks that used to be easy
The most common causes
Not enough sleep. The CDC recommends at least 7 hours a night for adults, and more than a third of US adults regularly get less. Sleep loss hits attention first, which is why a short night makes everything feel harder. Our guide to sleep and memory explains why sleep is also when the brain files away what you learned.
Stress, anxiety and depression. All three take up mental bandwidth and disrupt sleep. In one 2022 study of women in midlife, depression, not hot flashes, was the factor linked to weaker cognitive performance. See stress and memory.
Long COVID. A 2024 meta-analysis of 17 studies and more than 41,000 long COVID patients found that about 1 in 5 had brain fog or mental health symptoms 3 to 24 months after infection. Yale Medicine notes it often improves within 6 to 9 months, though some people have it for 18 months or more.
Hormonal changes. A 2018 meta-analysis of 20 studies found memory and thinking scores were measurably lower in the third trimester of pregnancy. In perimenopause, the long-running SWAN study suggests fog is usually temporary and improves after the transition.
Vitamin B12 deficiency. According to the NIH, low B12 can cause confusion, poor memory and depression, even without anemia. Risk is higher for adults over 50, vegans, people who’ve had stomach surgery, and people taking metformin or acid-reducing drugs such as omeprazole.
Thyroid problems. In a survey of more than 5,000 people with an underactive thyroid, many described brain fog as forgetfulness, sleepiness and trouble focusing, and over half noticed it before they were diagnosed.
Medications. Harvard Health lists sleep aids such as diphenhydramine and zolpidem, opioids, older tricyclic antidepressants and gabapentin as common culprits. Never stop a prescription on your own; bring a list of what you take to your doctor.
Cancer treatment. Cleveland Clinic estimates that 70 to 75% of people with cancer have thinking problems during or after treatment, often called “chemo brain”.
Other conditions. Autoimmune diseases such as lupus and MS, diabetes and low blood sugar, ADHD, and poor nutrition can all show up as fog.
How to clear brain fog
If there’s an underlying cause, treating it matters most. Alongside that, the basics recommended by Cleveland Clinic are dull but effective:
- Protect your sleep. Same bedtime and wake time, and no caffeine late in the day. Our guide to caffeine and memory covers how long it lingers.
- Move every day. About 30 minutes of activity. Even one workout gives a small, short-term boost to thinking; see exercise and memory.
- Eat regular, balanced meals. Low blood sugar alone can make thinking slow.
- Offload your memory. Lists, calendars and notes free up working memory for the task in front of you.
- Do one thing at a time. Switching tasks costs focus. See multitasking and the brain.
- Take real breaks from mental work, ideally away from screens.
Track how sharp you feel
Fog is hard to judge from the inside. A short, consistent check-in can help you notice patterns, like whether you’re sharper after a good night’s sleep. Try this quick visual memory test:
For a daily check-in, the Recall game takes about a minute and gives everyone the same puzzle each day, so your scores are comparable from one day to the next. It’s a game, not a medical test, but a run of unusually low scores can be a nudge to look at your sleep and stress.
When to see a doctor
Cleveland Clinic suggests seeing a doctor if fog affects your quality of life, makes you miss appointments or makes ordinary tasks hard. Yale Medicine recommends an evaluation if it lasts more than 3 months, to check for thyroid problems, vitamin deficiencies, sleep apnea and ADHD. Sudden confusion, trouble speaking, a drooping face or weakness on one side is a medical emergency, not brain fog: call emergency services.
This article is general information, not medical advice or a diagnosis. If you’re worried about your memory, thinking or health, talk to a doctor.
Sources
- Cleveland Clinic: Brain fog
- CDC (2016): 1 in 3 adults don’t get enough sleep
- van der Feltz-Cornelis et al. (2024), General Hospital Psychiatry: brain fog and mental health in long COVID
- Yale Medicine: How to manage long COVID brain fog
- Cleveland Clinic: Chemo brain
- Harvard Health: Menopause and brain fog
- Davies et al. (2018), Medical Journal of Australia: cognition during pregnancy, a meta-analysis
- NIH Office of Dietary Supplements: Vitamin B12
- American Thyroid Association: brain fog in hypothyroidism
- Harvard Health: Stuck in a brain fog? Look in your medicine cabinet
Frequently asked questions
What causes brain fog suddenly?
Common triggers include a run of poor sleep, high stress, a recent infection such as COVID-19, a new medication, and hormonal changes in pregnancy or perimenopause. Sudden confusion, trouble speaking or weakness on one side of the body is different: that’s an emergency, so call emergency services.
Can lack of sleep cause brain fog?
Yes. Sleep loss hits attention and alertness hardest. The CDC recommends at least 7 hours a night for adults, and more than a third of US adults regularly get less.
What vitamin deficiency causes brain fog?
Low vitamin B12 can cause confusion, poor memory and low mood, even without anemia. Adults over 50, vegans, and people taking metformin or acid-reducing drugs are at higher risk. A blood test can check it.
How long does long COVID brain fog last?
It often improves within 6 to 9 months, but some people have it for 18 months or longer. Yale Medicine suggests getting checked if it lasts more than 3 months.
How do I get rid of brain fog?
Treat the cause if there is one, then cover the basics: regular sleep, daily movement, regular meals, breaks from mental work, and writing things down so your memory isn’t carrying everything.