Brain fog can be caused by a variety of factors that include mental stress, sleep deprivation and low levels of vitamin B-12 and magnesium in the body, according to Nutrex Hawaii. Additional causes of brain fog include hormone fluctuations due to menopause and side effects of certain medications.
Individuals with fibromyalgia also suffer from a type of brain fog referred to as fibro fog, according to the Arthritis Foundation. The specific cause of brain fog in fibromyalgia sufferers is not completely understood, but certain studies suggest the symptom may be due to a lack of oxygen in the brain. Fibro fog may also occur when fibromyalgia sufferers experience severe pain.