Brain fog and mental fatigue: causes, symptoms and what to do

Brain fog is that feeling of having cotton wool in your head: you search for words, you read the same sentence three times, and mental fatigue quickly sets in. It is not a diagnosis but a symptom, and it almost always has an explanation. Here are the common causes, the signs that should prompt you to see a doctor, and what science really knows about the link between brain energy, NAD+ and vitamin B3.

Brain fog: what exactly are we talking about?

Brain fog, sometimes called mental fog or cognitive fog, describes a perceived drop in mental clarity. No test can “measure” it: the term describes an experience, not a cause.

In 2023, researchers at the University of Edinburgh analysed 717 social media posts containing the expression. People mainly described forgetfulness, difficulty concentrating, a sense of detachment, slow thinking and fatigue. The cause most often mentioned was long COVID (10% of the posts that named a cause). The conclusion: the same word covers very different realities [1].

What are the symptoms of brain fog?

• difficulty concentrating or following a conversation;
• everyday forgetfulness (keys, appointments, what you were about to say);
• a sense of thinking in slow motion, of having to “force” it;
• mental fatigue that sets in quickly during a demanding task;
• a heavy or “woolly” head.

These signs often fluctuate during the day. A genuine decline in memory, on the other hand, gets steadily worse: it warrants prompt medical advice.

What are the common causes of brain fog?

There is rarely just one cause.

Lack of sleep

This is the most ordinary cause. A meta-analysis of 70 studies shows that total sleep deprivation of less than 48 hours mainly impairs simple attention, with many attention “lapses”, while reasoning is less affected [2]. Snoring and waking unrefreshed despite long nights may point to sleep apnoea.

Chronic stress and cortisol

Ongoing stress keeps the body on alert, disrupts sleep and fills your mental “bandwidth” with rumination. To get a clear picture of this hormone, read our article on cortisol and the myths around it.

Perimenopause and menopause

In the American SWAN study, 2,362 women were followed for 4 years. During perimenopause, their scores (processing speed, verbal memory) stopped improving from one test to the next. After menopause, they returned to their previous level: these difficulties appear to be temporary [3].

Long COVID

A meta-analysis of 81 studies estimates that 32% of people still have fatigue and 22% cognitive impairment 12 weeks or more after a COVID diagnosis [4].

Other possible causes

Iron or vitamin B12 deficiency, an underactive thyroid, unstable blood sugar, not drinking enough water or a medication can also cloud your thinking. A meta-analysis of 33 studies shows that dehydration mainly impairs attention and executive function, especially beyond a loss of 2% of body weight [5].

For the thyroid, see our article on iodine deficiency. If your energy crashes follow meals, read our guide to insulin resistance.

When should you see a doctor?

See your GP if brain fog lasts more than a few weeks, gets worse or interferes with your daily life, if those close to you notice unusual forgetfulness, or if it began after a blow to the head or a new treatment.

Call the emergency services immediately (112 across Europe, 15 in France) in the event of sudden confusion, sudden difficulty speaking, weakness on one side of the body or a severe, unusual headache.

Routine tests: full blood count and ferritin, vitamin B12, TSH, fasting blood glucose or HbA1c, vitamin D. They rule out the causes that are easiest to correct.

Why is the brain so sensitive to a lack of energy?

In adults, the brain accounts for about 2% of body weight but uses about 20% of the body's oxygen, and therefore of its calories [6]. Neurons therefore depend on their power plants, described in our article on the role of mitochondria.

This is where NAD+ comes in. This coenzyme, made in particular from vitamin B3, carries the electrons that allow mitochondria to produce ATP, the energy “currency” of cells. Its levels fall with age in several tissues (see NAD+: definition, roles and decline with age). Hence the value of knowing how to support it.

Nicotinamide riboside and the brain: what do human trials show?

The short answer: NR reliably raises NAD+, from the dose of a single capsule.

• Healthy overweight adults (2019). Over 8 weeks, 300mg of NR a day raised blood NAD+ by 51% within 2 weeks, with no flushing reported [8].
• Middle-aged and older adults (2018). In a 2 × 6-week crossover trial, NR was well tolerated and raised NAD+ [7].

In plain terms: NR reliably raises blood NAD+. What an NR supplement can claim rests on vitamin B3: NR is a form of vitamin B3, and EU law notably authorises “Niacin contributes to normal psychological function.” and “Niacin contributes to normal functioning of the nervous system.” [9].

What else can help you think clearly again?

• Sleep. Regular hours, a cool and dark bedroom, less screen time in the evening: this is the most powerful lever.
• Physical activity. Brisk walking, cycling or strength training several times a week are associated with better cognitive health. If you have long COVID, increase your effort very gradually, with your doctor.
• Water and meals. Drink regularly. Favour protein, fibre and vegetables over fast sugars that make blood sugar spike and then crash.
• Magnesium. Magnesium contributes to a reduction of tiredness and fatigue. Magnesium contributes to normal psychological function. For the form, see our comparison of magnesium forms; L-threonate is the form used in Magtein® Hexagon® (€34.90).
• Vitamin B12. Vitamin B12 contributes to normal functioning of the nervous system. Vitamin B12 contributes to the reduction of tiredness and fatigue. Vegans and older adults are more likely to fall short.

Frequently asked questions

Are brain fog and mental fatigue the same thing?

Not quite. Mental fatigue is exhaustion in the face of intellectual effort; brain fog is a lack of clarity. The two often go together and share the same causes.

Which vitamin helps with brain fog?

If a blood test reveals a B12 or iron deficiency, correcting it is the priority, under medical supervision. As for vitamin B3, “Niacin contributes to normal psychological function.”; NR is one of its forms. Vitamin D, for its part, has no claim related to fatigue.

Can a food supplement help with brain fog?

It can top up an insufficient intake, but it does not replace finding the cause. Choose clearly stated doses and talk to your doctor if you are on medication.

Can stress or anxiety cause brain fog?

Yes, very often: anxiety monopolises attention. If lasting sadness or a loss of interest come on top of it, talk to a doctor.

How long does brain fog last?

It all depends on the cause: one to two days after a bad night, a few months or more after COVID. If it lasts beyond a few weeks with no explanation, see a doctor.

Who should I see about brain fog?

Your GP first. Depending on the likely cause, they will refer you to a neurologist, a gynaecologist, an endocrinologist, a sleep specialist or a psychologist.

Conclusion

Brain fog is a signal, not a diagnosis. Start with the basics, and with a check-up if it persists. NAD+ is central to neuron energy, and NR raises it in the blood. If you would like a source of vitamin B3 in the form of NR, Hexagon® NAD+ provides 300mg per capsule.

Sources

1. McWhirter L, Smyth H, Hoeritzauer I, et al. What is brain fog? J Neurol Neurosurg Psychiatry. 2023;94(4):321-325.
2. Lim J, Dinges DF. A meta-analysis of the impact of short-term sleep deprivation on cognitive variables. Psychol Bull. 2010;136(3):375-389.
3. Greendale GA, Huang MH, Wight RG, et al. Effects of the menopause transition and hormone use on cognitive performance in midlife women. Neurology. 2009;72(21):1850-1857.
4. Ceban F, Ling S, Lui LMW, et al. Fatigue and cognitive impairment in Post-COVID-19 Syndrome: a systematic review and meta-analysis. Brain Behav Immun. 2022;101:93-135.
5. Wittbrodt MT, Millard-Stafford M. Dehydration impairs cognitive performance: a meta-analysis. Med Sci Sports Exerc. 2018;50(11):2360-2368.
6. Raichle ME, Gusnard DA. Appraising the brain's energy budget. Proc Natl Acad Sci U S A. 2002;99(16):10237-10239.
7. Martens CR, Denman BA, Mazzo MR, et al. Chronic nicotinamide riboside supplementation is well-tolerated and elevates NAD+ in healthy middle-aged and older adults. Nat Commun. 2018;9(1):1286.
8. Conze D, Brenner C, Kruger CL. Safety and metabolism of long-term administration of nicotinamide riboside chloride in a randomized, double-blind, placebo-controlled clinical trial of healthy overweight adults. Sci Rep. 2019;9(1):9772.
9. Commission Regulation (EU) No 432/2012 of 16 May 2012 establishing a list of permitted health claims made on foods, other than those referring to the reduction of disease risk and to children's development and health.

Food supplements should not be used as a substitute for a varied and balanced diet and a healthy lifestyle. Ask a healthcare professional for advice if you are on medication, pregnant or breastfeeding.