Brain fog isn’t just forgetfulness: What causes it? Is it linked to dementia? When should you worry?

Some days are better than others, to reference a U2 song. Who hasn’t had a moment (or a few) when your brain simply says “no”? You lose your train of thought, can’t seem to focus, or have trouble finding the right word, phrase, term… you get the drift.

You chalk up the proverbial “brain fog” to a lack of sleep or the particularly stressful period you’re in. Perhaps it’s menopause beckoning from the hormonal pit that ageing has put you in. Or maybe, it’s a remnant from COVID-19. Wait, could it have been dengue? 

“Defining brain fog is challenging due to the absence of a single universally accepted definition,” said Dr Jillian Teo, a senior psychologist from National Neuroscience Institute’s (NNI) Department of Psychology. 

In general, she continued, brain fog refers to “a cluster of overlapping symptoms involving cognition, fatigue and mood”. So yes, it’s a mishmash of the abovementioned signs, plus difficulties in your processing speed. 

“Some individuals may also report mental fatigue, confusion, or feeling overwhelmed by an overload of information,” said Dr Teo.

“Less obvious accompanying difficulties can include irritability, reduced tolerance for multitasking, making more everyday errors, and taking longer to complete familiar tasks,” said Dr Huang Wan Ping, the senior principal psychologist at Raffles HealthyLongevity. 

“People may say, ‘my brain feels slow’, ‘I can’t think straight’, or ‘I have to read the same paragraph several times’.”

WHAT CAUSES BRAIN FOG?

Dr Huang highlighted that “sleep is an especially important and sometimes underestimated contributor”. “Sleep loss can affect attention, working memory, inhibitory control and cognitive flexibility, which depend substantially on networks involving the frontal regions of the brain.”

According to Dr Teo, “brain fog is frequently reported across a range of conditions and can arise from many different causes”, including the following:

• Infections and inflammation
• Nutritional deficiencies, such as being low on Vitamin B12 or iron
• Hormonal changes such as during peri- and menopause, thyroid problems
• Changes in brain chemicals, such as acetylcholine, dopamine and noradrenaline, that affect attention and alertness
• Poor and disrupted sleep, including jetlag
• Stress
• High and low blood sugar levels
• Physical exhaustion and fatigue
• Alcohol consumption
• Traumatic brain injury 
• Substance abuse 

WHAT’S HAPPENING IN THE BRAIN? 

“The prefrontal cortex (front part of the brain) and frontoparietal network (the brain’s cognitive control centre) are particularly important because they help us maintain attention, manipulate information, prioritise relevant information, and suppress distractions,” explained Dr Huang. 

“When the brain is sleep deprived, fatigued, physiologically stressed or unwell, the efficiency of these systems can decrease. The person may still possess the knowledge or memory but accessing it can become slower and more effortful,” said Dr Huang. 

However, he hesitated to pin brain fog on any singular neuron or neurotransmitter. “There is currently no single neurotransmitter, brain region or biological mechanism that explains all experiences of brain fog. Thinking clearly requires several brain systems to work together efficiently,” said Dr Huang. “I would describe brain fog less as ‘neurons malfunctioning’ and more as the brain temporarily becoming less efficient at allocating cognitive resources.”

HOW LONG DOES BRAIN FOG USUALLY LAST?

The experts don’t have a specific answer. “The severity, frequency and duration of brain fog vary considerably across individuals, depending on the cause,” said Dr Teo.

For instance, “sleep-related brain fog may improve after restorative sleep, while stress-related symptoms may fluctuate over days or weeks”, explained Dr Huang. “Cognitive symptoms associated with conditions such as long COVID-19 can persist for months or longer. Brain fog can also fluctuate according to sleep, fatigue, illness, stress, and cognitive demand.”

BESIDES COVID-19, CAN OTHER VIRAL INFECTIONS ALSO CAUSE BRAIN FOG?

Much has been said about long COVID-19 symptoms – one of which is the infamous brain fog. But before the pandemic hit, experts have already suspected that brain fog might also be seen in other viral infections such as dengue.

In fact, a 2024 study by Nanyang Technological University (NTU) researchers found that those who contracted dengue faced a 213-per-cent increased risk of cognition or memory disorders. The study had looked at tests and the medical claim records of 11,707 Singapore residents with dengue between July 2021 and October 2022.

To also examine the dengue-and-brain connection, Singapore General Hospital’s Dr Shirin Kalimuddin, a senior consultant from Department of Infectious Diseases, recruited 209 patients with acute viral infections such as dengue and followed them for up to a year – right before the pandemic struck in 2019. Of the lot, 48 patients had dengue. 

ARE THERE FOODS THAT ELIMINATE BRAIN FOG?

There are no specific foods that have been proven to prevent or eliminate brain fog, said Dr Jillian Teo, a senior psychologist from National Neuroscience Institute’s Department of Psychology. 

What good nutrition does is reduce neuroinflammation and support the gut-brain axis, which is a two-way communication network that links your central nervous system with your digestive tract, she said. To do that, it helps to keep a balanced and nutrient-dense diet rich in fibre, omega-3 fatty acids, antioxidants, and minimally processed foods.

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The results confirmed the link as well. In a paper published in 2022, Dr Shirin noted that almost all of the patients (95.8 per cent of dengue patients; 94 per cent of patients with other acute viral respiratory infections) “experienced at least one symptom of fatigue, somnolence (a state of extreme drowsiness), headache, concentration impairment or memory impairment within the first week of enrolment”. 

The SGH study showed that those “neurological symptoms that persist can also be an issue after other infections, such as dengue”, said Dr Nicole Keong, a senior consultant and neurosurgeon at Department of Neurosurgery, NNI. 

“Symptoms may include memory difficulties, concentration, thinking speed, fatigue, balance, or changes in or loss of the sense of smell – and they can have a long-term impact on daily activities and quality of life.”

But as to how dengue causes brain fog, “the mechanism for this is still unknown”, said Dr Keong. “It is also unclear how long brain fog and related symptoms last for.”

To study the effects dengue has on the brain, Dr Keong and her team are currently recruiting dengue patients for the NEurodegeneration & Understanding of Risk Overlay in Neuroimaging (NEURON3) study. To date, they have found “altered brain signatures” in dengue patients in the chronic stage. “However, it is too early in our study to be able to interpret what those changes mean,” said Dr Keong.

“By studying the impact of these common infections on our brain health, we aim to contribute to a deeper understanding of how the brain responds to and recovers from threats to its functioning, following infection and injury,” said Dr Keong. 

WHAT ABOUT MENOPAUSE?

“In women experiencing perimenopause, declining oestrogen levels can contribute significantly to experiences of brain fog,” said Dr Teo.

Oestrogen helps support glucose metabolism, synaptic function and the balance of neurotransmitters, she explained. “As oestrogen levels decrease, the brain becomes less efficient at glucose metabolism, particularly in regions such as the hippocampus and prefrontal cortex that are critical for memory, attention, and executive functioning.”

The good news is, the cognitive complaints that many women have during the menopausal transition often stabilise following menopause, said Dr Teo. “However, some women continue to report difficulties with memory, attention and processing speed, with such complaints being more frequently reported among postmenopausal than premenopausal women.”

DOES HAVING HAD BRAIN FOG MEAN YOU ARE MORE PRONE TO DEMENTIA?

“Brain fog itself has not been established as a cause of dementia,” said Dr Huang. “Someone experiencing transient brain fog because of sleep deprivation, stress or illness should not conclude that they are developing dementia.”

Rather, “dementia risk is influenced by a range of established factors, including age, cardiovascular and metabolic health, genetics, lifestyle factors, and underlying brain pathology”, said Dr Teo.

While brain fog and dementia can share some symptoms, particularly forgetfulness and difficulty concentrating, the clinical patterns can be very different, according to Dr Huang. “Brain fog often fluctuates. People commonly remain aware of their cognitive difficulties, and performance may vary according to fatigue, stress or sleep.” 

Nevertheless, said Dr Teo, persistent and progressive cognitive changes should not be dismissed as "just brain fog". Dementia that occurs in the 40s and 50s may present atypically, with symptoms progressively worsening over months to years despite management, she said. 

Look out for significant language or word-finding difficulties; personality or behavioural changes; declining ability to manage activities in daily living; and a family history of young-onset dementia, advised Dr Teo.

CAN EXERCISE HELP WITH BRAIN FOG?

“Exercise may not ‘clear’ brain fog immediately but regular physical activity can support brain and body function, helping with focus, energy and coordination,” said senior physiotherapist Choo Kai Bing from 365 Cancer Prevention Society.

Try activities that challenge both mind and movement. Dancing is a good option as you have to remember sequences and coordinate your steps. Or try pickleball, where you have to track the ball and react quickly. “These activities help strengthen the connection between what your brain sees and processes, and how your body responds,” said Choo.

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The same goes for women with menopause-related brain fog. Cognitive complaints, such as difficulties with attention, memory and concentration, can impact performance at work, social relationships, confidence and overall wellbeing, said Dr Teo.

To sum it up, “the sensible response to persistent brain fog is not to assume dementia”, said Dr Huang. “But neither should we simply ignore it; a medical assessment can help identify and address the underlying cause.”

WHAT CAN YOU DO TO MANAGE BRAIN FOG?

Here’s what the experts suggest you can do to tackle the various cognitive issues that come with brain fog:

• Trouble focusing and organising your thoughts 

Avoid multitasking. Instead, do one task at a time, suggested Sarah Zheng, a counsellor from 365 Cancer Prevention Society. If it’s a big task, break it into smaller steps – and give yourself time to complete each step, she said. 

To help you further, Dr Huang recommended reducing your environmental distractions, such as closing unnecessary tabs on your browser and silencing your notifications.

• Poor memory

“Attention is the gateway to memory,” said Dr Huang. “If you are replying to messages while in a meeting, and thinking about your next appointment, information may never be properly encoded. This can create a cycle where we become convinced that ‘my memory is terrible’ when the underlying issue may be that we were not able to give the information our full attention in the first place.”

Sometimes, improving attention at the point of learning is a better strategy, said Dr Huang. “When we want to achieve more, it can help to start with doing less. Be present and focus on one thing at a time.”

• Slow at processing new information 

Tell yourself that it is okay to take more time to process the information and respond, said Zheng. “Avoid rushing important decisions. Note down important information if needed.” 

Dr Teo suggested using external memory aids such as calendars, reminder apps, and checklists to minimise your cognitive load of holding large volumes of information. Mnemonic techniques (such as using acronyms, stories, or rhymes), visualisation, or chunking information into smaller, more manageable pieces may also support learning and recall, she added.

And no, you’re not copping out or “giving in” to brain fog when you use these aids, said Dr Huang. “In neuropsychological rehabilitation, we call these compensatory strategies, which means using the environment to reduce cognitive load.”

• Difficulty finding a word

Allow yourself extra time to retrieve the word without panicking, advised Dr Teo. This can be helpful as stress can further interfere with word retrieval. “Strategies such as describing the concept, paraphrasing, or returning to the word later can often be helpful.”

• Mental exhaustion

If you find yourself mentally powering down during office hours, especially in the afternoon, naturopath Sebastian Liew of Leaf To Life, suggested running cool water from your elbows down your forearms as well as applying cool water to your nape to recharge. “Do not towel dry; just wipe the excess,” he said. It also helps to walk away from your desk for a while to break the stagnant energy, he said.

For an afternoon snack, opt for an apple for better blood sugar control, rather than sugar-loaded biscuits and sweets that spike and crash your blood sugar levels, Liew said.

• Struggling to wind down after work

“If it is not possible to screen-fast because of a late assignment, take a cool lower body shower after work,” suggested Liew. Again, do not towel dry but wipe excess water only. 

To help you calm your mind and get ready for bed, Liew suggested diaphragmatic breathing:

• Sit comfortably in your bed with your back straight.
• Close your eyes and inhale for four counts (your belly should rise). Slowly exhale for eight counts (your belly should fall). 
• If any stressful moment or image comes to your mind, admit the resulting emotion to yourself by thinking “I am angry/worried/frustrated" etc. Don’t suppress or deny your feelings. 
• As you breathe, recall and visualise someone, something or somewhere that regenerates positive feelings of gratitude or love. The main thing is to recreate the feeling, not intellectual thinking. Or you can recall moments of your creativity, laughter and joy. Stay with the feeling for at least two minutes.