“Brain fog” is a symptom description rather than a diagnosis: reduced focus, slowed thinking, forgetfulness, and lowered mental performance. Common causes include poor sleep, stress, heavy screen/workload, suboptimal diet, and blood-sugar swings. Medical contributors can include iron/B12 deficiency, thyroid disorders, mood conditions, medication effects, post-infection states, and sleep apnea. First-line management focuses on sleep quality, regular walking, protein+fiber nutrition, and structured breaks. If it lasts beyond a few weeks, impairs function, or comes with neurologic red flags, seek clinical evaluation.