Quoted from Jigsaw Health See their site for source and more info

Brain fog may be the result of:

• Adrenal fatigue syndrome – The adrenal glands fail to produce key hormones that effectively regulate the body’s stress response. As a result, adrenal fatigue syndrome can cause brain fog symptoms due to the body’s inability to handle stress effectively.
• Insomnia / sleep deprivation – Sleep is crucial for optimal brain function. Lack of adequate sleep can affect mood, cause depression, anxiety, and clouded thinking.
• Chronic fatigue syndrome (CFS) – Chronic fatigue syndrome is known for its overwhelming fatigue. CFS affects concentration, short-term memory and disrupts healthy sleep patterns.
• Nutritional deficiencies – Nutritional deficiencies can affect brain function, due to a lack of nutrients necessary for optimal brain function. Deficiencies of magnesium, vitamin B-12, and amino acids can cause significant brain impairment such as lack of concentration, short-term memory loss, attention deficit, and spaciness (or lack of focus).
• Candidiasis – Systemic yeast overgrowth can cause depression, anxiety, sudden mood swings, lack of concentration, headaches, drowsiness, and/or fatigue.
• Autoimmune disorders such as fibromyalgia or multiple sclerosis (MS) – Fibromyalgia and MS are both nervous system disorders that affect brain function, leading to, in many cases, overwhelming fatigue, depression, and anxiety.
• Chronic viral infections such as Epstein-Barr, Lyme disease, hepatitis, cytomegalovirus – Viral infections can reach the brain and can significantly disrupt thinking and mood.
• Parasitic organisms found in the brain - Parasitic infections such as tapeworm can invade the brain and cause symptoms of acute mental illness such as depression and psychosis in more than 65% of cases. Parasitic infections outside of the brain that can produce psychiatric symptoms include giardia, ascaris psychosis, trichinosis, and Lyme Disease, among many others.
• Heavy metal toxicity - The most common heavy metals that humans are exposed to are aluminum, arsenic, cadmium, lead, and mercury. These often accumulate in brain tissue and are difficult to excrete in many cases. Some people—especially those who suffer from chronic conditions—cannot excrete neurotoxic heavy metals efficiently and a build-up occurs, causing brain fog symptoms.
• Reduced blood flow to the brain due to circulatory problems – Lack of blood flow to the brain disrupts brain function. Thick blood can also slow down blood flow to the brain, causing brain dysfunction.
• Blood sugar issues such as hypoglycemia or diabetes – Uncontrolled blood sugar levels can cause significant symptoms such as depression, anxiety, fatigue, and clouded thinking.
• Overuse of artificial sweeteners and MSG – Artificial sweeteners such as Aspartame and flavor enhancers are considered excitotoxins, meaning that they are toxic to the brain, and can disrupt brain function.
• Allergies and food intolerances such as to gluten and lactose – Many patients who are lactose and/or gluten intolerant find that their brain function often suffers as a result of allergies to these substances.
• Leaky gut syndrome – When the gut becomes more permeable, larger particles escape into the bloodstream and may pass the blood brain barrier, causing a wide variety of mental symptoms such as mood disturbance, depression, anxiety, fatigue, lack of concentration and focus, and short-term memory loss.
• Side effects of prescription and over-the-counter medications – It’s always important to double check with your pharmacist for medication side effects that can disrupt brain function. Many medications have side effects that disrupt mental health and may cause symptoms ranging from depression, anxiety and agitation, to severe conditions such as psychosis and suicidal ideation.
• Constipation – Brain fog may be caused by bowel toxicity due to inefficient digestive system that is not able to effectively eliminate toxins from the body.
• Menopause – Many women experience brain fog due to fluctuating hormone levels during perimenopause and menopause. Lowered estrogen levels may cause mood fluctuations and unclear thinking, depression, and anxiety.
• Sick building syndrome (fumes from new carpets, paints, insulation, chipboard, wood treatments, pesticides, carbon monoxide, poor ventilation) – Chemicals used during construction and for building fumigation may cause significant brain impairment in individuals sensitive or allergic to these chemicals.