What Is Brain Fog? Brain fog is a common but often misunderstood condition described as a feeling of mental cloudiness, difficulty concentrating, memory lapses, slowed thinking, or reduced mental clarity. It is not a medical diagnosis on its own but rather a symptom associated with factors such as stress, poor sleep, neuroinflammation, chronic illness, hormonal changes, mitochondrial dysfunction, or post-viral syndromes. Understanding the Brain–Energy Connection Healthy brain