Five families cause most of it, and the two that get missed most are the interesting ones. Iron can be empty while your blood test reads normal. The standard count checks hemoglobin, but ferritin (your storage iron) runs dry long before hemoglobin drops, and an empty tank starves the enzymes your muscles and brain run on. In non-anemic women with ferritin under 50, iron cut fatigue nearly in half ¹.

B12 is the second blind spot. A serum result inside the normal band can still mean your cells can't use the vitamin; a follow-up called methylmalonic acid catches what it misses ². Two of the world's most-prescribed drugs drain it slowly: metformin ³ and the acid-blocking PPIs like omeprazole ⁴.

Sleep apnea is your airway collapsing dozens of times an hour. It hits roughly one in ten men in their thirties and forties, more with age ⁵, and women get missed for years because they show up tired and sleepless instead of snoring. Overt low thyroid resolves once it's replaced ⁶. Fatigue is a core symptom of depression and its most stubborn leftover ⁷. And 11% stay fatigued six months after a viral illness ⁸.

One blood draw first, then a sleep test if it's clean. Ask for the actual numbers, and read the borderline ones against the trial thresholds, not the lab's "normal" range.

How fast it lifts depends on which test lit up.

• Iron: four to twelve weeks once ferritin clears the threshold, fatigue nearly halving by week twelve in non-anemic women ¹.
• B12 and thyroid: energy and focus return within weeks, though nerves recover slower ².
• CPAP for apnea is the dramatic one: a few awkward nights, then you wake before the alarm and realise you hadn't slept in years.
• Post-viral fatigue is the slow case: most recover over months, some don't, and the win is a name and the right strategy ¹⁰.