For how we live.
"Brain fog" is a lay term for trouble concentrating and remembering, not a disease. Common drivers include poor sleep, low mood, the menopause transition, thyroid or B12 problems, and the tail of an infection.
When adults see a doctor for new fatigue, fewer than one in ten turn out to have clear physical disease. The common, treatable and often-missed exception is iron deficiency, which drains energy before it causes anaemia.
Feeling cold when others are comfortable can point to an under-active thyroid or iron-deficiency anaemia. Fingers and toes that turn white then blue in the cold are usually Raynaud's, a blood-vessel spasm.
That heavy, foggy feeling on waking is often sleep inertia — a normal, temporary state, not a sign you slept badly. It bites hardest when you wake from deep sleep or during the biological night.
In people who are iron-deficient without anaemia, supplements ease self-reported fatigue in trials — but do nothing for objective exercise capacity, and the benefit hinges on being iron-deficient, not just tired.
In a US cohort followed eight years, average weekday sleep rose from 8.1 to 8.3 hours while reports of fatigue and trouble sleeping increased. The socioeconomic gap in duration closed; the gap in symptoms did not.