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Always tired? The causes in order
Sleep debt, iron, thyroid, vitamin D, mood, apnoea, medication, alcohol — the causes of persistent tiredness in the order a clinician works through them, each with the one check that rules it in or out.
The list, in order
| Order | What it is | Kind |
|---|---|---|
| 1. Sleep debt | The most common cause, and the easiest to fix or rule out. | common cause |
| 2. Iron deficiency | The common deficiency cause of tiredness, especially with heavy periods or a low-meat diet. | common cause |
| 3. Underactive thyroid | Slow, gradual tiredness with weight gain, coldness and dry skin — often mistaken for ageing or mood. | common cause |
| 4. Vitamin D | Low levels are common at these latitudes, but D is rarely the whole story on its own. | common cause |
| 5. Mood: depression and stress | Tiredness is one of the two classic physical symptoms of depression — the other is broken sleep. | red flag |
| 6. Sleep apnoea | Loud snoring with breathing pauses: you sleep all night and wake up unrefreshed. | red flag |
| 7. Medication and alcohol | The two causes people forget to count as causes. | common cause |
| 8. When tiredness means see a doctor | The signs that move this from lifestyle to medical. | red flag |
How to use this list
Work top to bottom, and rule each one out before moving on — the top three account for most of the tiredness a clinician sees. Each page says what to check first, because the point is a decision, not a list: two weeks of fixed sleep answers the first cause for free, a ferritin and TSH test answers the next two in one draw, and an honest two-week alcohol break answers another.
Reference, not medical advice. Progressive, unexplained tiredness — especially with weight loss, fevers, night sweats, breathlessness or new headaches — deserves a clinician, not this page. Nothing here diagnoses.
Sources: NICE CKS — tiredness/fatigue in adults · NHS — feeling tired all the time · checked 2026-09-07