6 Reasons for Brain Fog You May Not Know About

You know the feeling. You walk into a room and forget why you're there. You re-read the same paragraph three times and none of it sticks. A word you use every single day suddenly won't come. Your thoughts feel like they're arriving through static.

This is brain fog, and if you've been dealing with it, you're not imagining it, and you're not alone.

Brain fog affects more than 25% of adults in the general population (Haywood, Rossell & Hart, 2025). It shows up after illness, during high-stress periods, following a knock to the head, in the hormonal shifts of midlife, and as a symptom of conditions like ADHD and depression. It's one of the most commonly reported cognitive complaints, yet it remains one of the most misunderstood - partly because it doesn't show up on a standard blood test, and partly because it looks different for everyone.

Understanding the reasons for brain fog is the first step to doing something about it. This article walks through the most common causes, what's actually happening in the brain, and what the research says about effective approaches.

What Is Brain Fog?

Brain fog isn't a medical diagnosis but rather a term that describes a cluster of cognitive symptoms affecting how you think, remember, focus, and process information. Common experiences include (Cleveland Clinic, 2024):

• Difficulty concentrating, even on simple tasks
• Forgetting words mid-sentence
• Lapses in memory
• Slow reaction time and thought processing
• Mental exhaustion that rest doesn't fix
• Difficulty paying attention

These symptoms can be mild and occasional, or persistent enough to interfere with work, relationships, and daily life. What they share is that they reflect a brain not running at its best and in most cases, there's an identifiable reason why. Here are 6 reasons why you may have brain fog.

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1. Brain Fog and Lack of Sleep

Sleep is when the brain does most of its essential maintenance - clearing metabolic waste through the glymphatic system, consolidating memories, and restoring the energy stores neurons need to function. When sleep is cut short or chronically poor in quality, this process is interrupted.

The result is a brain running on depleted resources. Reaction time slows, working memory capacity drops and the prefrontal cortex - responsible for focus, decision-making, and filtering irrelevant information - becomes measurably impaired (Ocon, 2013). This is why even a single night of poor sleep can produce symptoms that feel almost identical to brain fog from other causes.

Common sleep disruptors that cause brain fog

• Insomnia: difficulty falling or staying asleep, leaving the brain under-restored.
• Sleep apnea: breathing interruptions fragment sleep cycles and reduce oxygen delivery to the brain overnight.
• Shift work: forces sleep out of alignment with the body's circadian rhythm, degrading sleep quality even when total hours are adequate.
• Jet lag: disrupts the internal clock, temporarily impairing cognitive function.
• Excessive screen exposure before bed: suppresses melatonin and delays sleep onset.

2. Brain Fog and Perimenopause

Brain fog and perimenopause are closely linked and yet the connection is still often dismissed or attributed to stress, age, or anxiety. For many women, cognitive symptoms are among the earliest and most disruptive signs that hormonal change is underway.

Estrogen plays an active and well-documented role in brain function. It supports memory, processing speed, and the health of the neurons themselves (Hara et al., 2014). As estrogen levels begin to fluctuate and decline during perimenopause - often years before the final menstrual period - many women notice changes in how their mind works.

What brain fog and perimenopause looks like

• Word-finding difficulties that weren't present before
• Short-term memory lapses: forgetting why you walked into a room, or losing the thread of a conversation
• Difficulty concentrating on tasks that previously felt effortless
• Mental fatigue that arrives earlier in the day
• A general sense of 'not being as sharp' that's hard to explain to others

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These symptoms are not imagined and not inevitable features of aging - they're physiological responses to changing hormonal conditions. The prefrontal cortex and hippocampus, both central to memory and focus, are particularly sensitive to estrogen fluctuations.

Sleep disruption during perimenopause driven by night sweats, anxiety, and hormonal shifts, further compounds cognitive symptoms. The two conditions reinforce each other, which is why brain fog during this life stage can feel particularly persistent and multi-layered.

3. Brain Fog and ADHD

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Brain fog and ADHD share significant overlap but hey're not the same thing, and understanding the distinction matters for finding the right approach.

ADHD involves structural and functional differences in how the brain's prefrontal cortex manages attention, working memory, impulse control, and executive function (Arnsten, 2010). People with ADHD often describe their experience as living with a permanent fog - a difficulty sustaining focus, organising thoughts, and completing tasks that don't respond to effort or willpower in the way that neurotypical minds expect.

How ADHD brain fog differs from other types

Where brain fog from sleep deprivation or illness tends to be temporary and tied to an identifiable cause, ADHD-related brain fog is typically consistent across contexts and present from an early age. It can be managed with the right strategies and support, but it doesn't simply lift when the person tries harder.

Key features of brain fog in ADHD include:

• Difficulty initiating tasks, even ones the person genuinely wants to do
• Working memory that feels like a whiteboard that gets erased too quickly
• Hyperfocus on engaging tasks, but rapid disengagement from routine ones
• Time blindness - poor awareness of how much time is passing
• Emotional dysregulation that increases mental fatigue

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ADHD is also commonly accompanied by sleep difficulties, anxiety, and depression - all of which can layer additional cognitive symptoms on top of the ADHD baseline, making the fog feel deeper than any single cause would produce alone.

For adults who have been living with undiagnosed ADHD, brain fog is often one of the primary complaints that eventually leads to assessment. If the fog is lifelong, inconsistently responsive to rest and lifestyle changes, and accompanied by difficulty with executive function, it's worth exploring whether ADHD may be a contributing factor.

4. Brain Fog and Concussion

Brain fog following a concussion is one of the most well-documented and most underestimated forms of cognitive impairment. Whether the injury came from a sporting collision, a fall, a car accident, or any other impact that causes the brain to move rapidly within the skull, the cognitive effects can be significant and lasting.

A concussion causes a cascade of neurological events: disruption to neurotransmitter systems, temporary impairment of cerebral blood flow, metabolic changes in neurons, and in some cases, stretching or shearing of axons - the fibres that connect brain cells (Giza & Hovda, 2001). The result is a brain that's physically disrupted, even when imaging often shows no visible structural damage.

Symptoms of brain fog after concussion

• Difficulty concentrating or following conversations
• Slowed thinking and processing speed
• Memory difficulties, particularly forming new memories
• Sensitivity to light and noise, which drains cognitive resources
• Fatigue that comes on rapidly with mental effort
• Headaches that worsen with cognitive activity

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For most people, concussion-related brain fog improves within days to weeks with appropriate rest. But for a significant minority - particularly those who return to activity too quickly, sustain repeated concussions, or have other risk factors - symptoms can persist for months or longer. This is sometimes called post-concussion syndrome.

Why it can last longer than expected

The brain's recovery from concussion depends heavily on its ability to restore cellular energy and reduce neuroinflammation. When this process is disrupted by poor sleep, ongoing physical activity too soon, or underlying vulnerability, the recovery window extends.

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5. Brain Fog and Depression

Depression is frequently mischaracterised as purely an emotional condition. In reality, it involves measurable changes in brain structure, chemistry, and function and brain fog is a common and debilitating feature (Trifu et al., 2020).

People with depression often describe cognitive symptoms as distinct from - and sometimes more impairing than - the emotional ones. The fog of depression can make previously routine tasks feel effortful, overwhelming, or simply impossible to start.

How depression affects cognition

Depression is associated with reduced activity in the prefrontal cortex, the region most responsible for focus, working memory, planning, and decision-making (Liu et al., 2017). It also affects the hippocampus, which is central to memory formation. Neuroinflammation - a consistent finding in depression research - further impairs how efficiently neurons communicate (Jeon & Kim, 2018).

Specific cognitive effects of depression-related brain fog include:

• Slowed information processing - thoughts that feel heavy and slow to arrive
• Difficulty concentrating, even on things the person is interested in
• Impaired working memory - holding information in mind long enough to use it becomes difficult
• Indecisiveness - the mental effort required for decisions feels disproportionate
• Negative cognitive bias - the brain defaults to worst-case interpretations, consuming additional resources

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Sleep is almost universally disrupted in depression, which compounds cognitive symptoms significantly. The relationship is bidirectional: poor sleep worsens depression, and depression worsens sleep, creating a cycle that deepens the fog.

The overlap with other causes

Brain fog and depression frequently co-occur with other causes on this list. Depression is common during perimenopause. ADHD and depression have high rates of co-occurrence. Long COVID includes depression as a frequent feature alongside cognitive symptoms. This layering is part of why brain fog in the context of depression can be particularly resistant to simple interventions.

6. Other Common Reasons for Brain Fog

Beyond the conditions above, several other causes are worth understanding, particularly because they're common, often overlooked, and frequently treatable.

Long COVID and post-viral conditions

Brain fog is one of the most persistent symptoms of long COVID, affecting memory, concentration, and processing speed for months or longer after initial infection. Neuroinflammation and disruption to the blood-brain barrier are among the leading hypotheses for why cognitive symptoms persist even when the acute illness has resolved (Greene et al., 2024).

Chronic stress

Sustained cortisol elevation - the hallmark of chronic stress - directly impairs prefrontal cortex function (McKlveen et al., 2017). Over time, chronic stress degrades the brain's ability to sustain attention, organise thoughts, and form new memories. It also disrupts sleep, compounding cognitive effects.

Nutritional deficiencies

Vitamin B12, iron, vitamin D, omega-3 fatty acids, and magnesium are all involved in brain function. Deficiencies in any of these can produce cognitive symptoms that closely resemble brain fog. A basic blood panel can identify most of them, and correction often produces rapid improvement.

Hormonal imbalances

Beyond perimenopause, thyroid dysfunction is a common and underdiagnosed cause of brain fog. Low thyroid function slows the metabolic rate of brain cells, producing fatigue, slow thinking, and memory difficulties (Ettleson et al., 2022). Insulin resistance can also affect cognitive clarity (Willmann et al., 2020).

Medications

Antihistamines, benzodiazepines, certain antidepressants, sleep medications, and chemotherapy are all associated with cognitive side effects (Risacher et al., 2017; Zetsen et al., 2022; Saczynski et al., 2017; Kovalchuk & Kolb, 2017). If brain fog coincides with starting or changing a medication, this connection is worth discussing with a prescribing doctor.

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What Can You Do About Brain Fog?

The right approach depends on the cause - which is why identifying the underlying reason matters. That said, several approaches have evidence across multiple causes.

Prioritise sleep

Regardless of the primary cause, sleep is almost always a factor. Addressing sleep quality - whether through sleep hygiene, treatment of sleep apnea, or support for insomnia - has cascading benefits for every other dimension of brain fog.

Rule out nutritional and hormonal causes

A basic blood panel can check for B12, iron, vitamin D, and thyroid function. These deficiencies are common, correctable, and often produce rapid cognitive improvement once addressed. For women in midlife, a conversation with a doctor about perimenopause and hormonal options is worth having early rather than late. These nutritional deficiencies are also common post head injury, and using supplementation as support can help with concussion-related brain fog.

Seek assessment for ADHD and depression

If brain fog is lifelong, or if it's accompanied by persistent low mood, loss of interest, or long-standing difficulties with attention and executive function, professional assessment is important. Both ADHD and depression are treatable conditions and untreated, they make other approaches to brain fog significantly less effective.

Manage concussion carefully

If brain fog follows a head injury, appropriate rest, a graduated return to activity, and professional monitoring are essential. Returning to full activity too quickly is one of the most common reasons post-concussion symptoms become prolonged.

Consider photobiomodulation

Near-infrared light therapy is one of the more promising emerging approaches for brain fog across multiple causes. Research has found improvements in working memory, reaction time, processing speed, sleep quality, and reductions in mood symptoms - with effects appearing within two to three sessions (Lundie et al., 2025).

The Neuronic LIGHT delivers 1070nm near-infrared light non-invasively to your neurons to help increase cellular energy, decrease inflammation and increase circulation - all factors contributing to brain fog.

The Bottom Line

Brain fog is real, measurable, and has identifiable causes. The most common reasons include lack of sleep, perimenopause, ADHD, concussion, and depression, as well as post-viral conditions, chronic stress, nutritional deficiencies, and hormonal imbalances. Often, more than one cause is operating at the same time, which is why the fog can feel so persistent and difficult to shift.

Understanding which causes apply to your situation is the most useful starting point. From there, addressing root causes alongside supporting brain energy and function directly - through sleep, nutrition, appropriate medical support, and tools like near-infrared light therapy - gives the brain what it needs to clear.

If you want to understand how near-infrared light therapy works and what the research shows, explore the Neuronic science library or speak to your healthcare provider about whether tPBM may be appropriate for you.

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References

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Arnsten, A. F. T. (2009). The emerging neurobiology of attention deficit hyperactivity disorder: The key role of the prefrontal association cortex. The Journal of Pediatrics, 154(5), I–S43. https://doi.org/10.1016/j.jpeds.2009.01.018

Cleveland Clinic. (2025, August 26). Brain fog: What it is, causes, symptoms & treatment. https://my.clevelandclinic.org/health/symptoms/brain-fog

Ettleson, M. D., Raine, A., Batistuzzo, A., Batista, S. P., McAninch, E. A., Teixeira, M. C. T. V., Jonklaas, J., Laiteerapong, N., Ribeiro, M. O., & Bianco, A. C. (2022). Brain fog in hypothyroidism: Understanding the patient’s perspective. Endocrine Practice, 28(3), 257–264. https://doi.org/10.1016/j.eprac.2021.12.003

Giza, C. C., & Hovda, D. A. (2001). The neurometabolic cascade of concussion. Journal of Athletic Training, 36(3), 228–235. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC155411/

Greene, C., Connolly, R., Brennan, D., Laffan, A., O’Keeffe, E., Zaporojan, L., O’Callaghan, J. P., Thomson, B., Connolly, E., Argue, R., Meaney, J. F. M., Martin-Loeches, I., Long, A., Ní Cheallaigh, C., Conlon, N., Doherty, C. P., & Campbell, M. (2024). Blood–brain barrier disruption and sustained systemic inflammation in individuals with long COVID–associated cognitive impairment. Nature Neuroscience, 27(3), 421–432. https://doi.org/10.1038/s41593-024-01576-9

Hara, Y., Waters, E. M., McEwen, B. S., & Morrison, J. H. (2015). Estrogen effects on cognitive and synaptic health over the lifecourse. Physiological Reviews, 95(3), 785–807. https://doi.org/10.1152/physrev.00036.2014

Haywood, D., Rossell, S. L., & Hart, N. H. (2025). Cutting through the fog: Recognising brain fog as a significant public health concern. BMC Public Health, 25, 1230. https://doi.org/10.1186/s12889-025-22525-6

Jeon, S. W., & Kim, Y. K. (2018). Neuroinflammation and neurovascular dysfunction in depression. Journal of Inflammation Research, 11, 179–192. https://doi.org/10.2147/JIR.S141033

Liu, W., Ge, T., Leng, Y., Pan, Z., Fan, J., Yang, W., & Cui, R. (2017). The role of neural plasticity in depression: From hippocampus to prefrontal cortex. Neural Plasticity, 2017, 6871089. https://doi.org/10.1155/2017/6871089

Lundie, M., Koch, M., Villanueva, A., Nguyen, T., Garner, C., Satsangi, M., Advani, P., & Krawczyk, D. (2025). Illuminating cognitive performance: Assessing the role of transcranial photobiomodulation in augmenting cognition. Military Medicine, 190(Supplement_2), 456–463. https://doi.org/10.1093/milmed/usaf227

McKlveen, J. M., Myers, B., & Herman, J. P. (2016). Chronic stress increases prefrontal inhibition: A mechanism for stress-induced prefrontal dysfunction. Biological Psychiatry, 80(10), 754–764. https://doi.org/10.1016/j.biopsych.2016.03.2101

Ocon, A. J. (2013). Caught in the thickness of brain fog: Exploring the cognitive symptoms of chronic fatigue syndrome. Frontiers in Physiology, 4, 63. https://doi.org/10.3389/fphys.2013.00063

Risacher, S. L., McDonald, B. C., Tallman, E. F., West, J. D., Farlow, M. R., Unverzagt, F. W., Gao, S., Boustani, M., Crane, P. K., Petersen, R. C., Jack, C. R., Jagust, W. J., Aisen, P. S., Weiner, M. W., Saykin, A. J., & Alzheimer’s Disease Neuroimaging Initiative. (2016). Association between anticholinergic medication use and cognition, brain metabolism, and brain atrophy in cognitively normal older adults. JAMA Neurology, 73(6), 721–732. https://doi.org/10.1001/jamaneurol.2016.0580

Saczynski, J. S., Rosen, A. B., McCammon, R. J., Zivin, K., Andrade, S. E., Langa, K. M., Vijan, S., Pirraglia, P. A., & Briesacher, B. A. (2015). Antidepressant use and cognitive decline: The Health and Retirement Study. The American Journal of Medicine, 128(7), 739–746. https://doi.org/10.1016/j.amjmed.2015.01.007

Trifu, S. C., Trifu, A. C., Aluaș, E., Tătaru, M. A., & Costea, R. V. (2020). Brain changes in depression. Romanian Journal of Morphology and Embryology, 61(2), 361–370. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7864313/

Willmann, C., Brockmann, K., Wagner, R., Kullmann, S., Preissl, H., Schnauder, G., Maetzler, W., Gasser, T., Berg, D., Eschweiler, G. W., Metzger, F., Fallgatter, A. J., Häring, H.-U., Fritsche, A., & Heni, M. (2020). Insulin sensitivity predicts cognitive decline in individuals with prediabetes. BMJ Open Diabetes Research & Care, 8(2), e001741. https://doi.org/10.1136/bmjdrc-2020-001741

Zetsen, S. P. G., Schellekens, A. F. A., Paling, E. P., Kan, C. C., & Kessels, R. P. C. (2022). Cognitive functioning in long-term benzodiazepine users. European Addiction Research, 28(5), 377–381. https://doi.org/10.1159/000525988

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