"Tired all the time" is so common that clinicians have an abbreviation for it. It is also genuinely worth investigating, because the causes are usually ordinary, usually identifiable, and usually fixable. Working through them in order beats guessing.
1. Not enough sleep, or not enough good sleep
The obvious answer is the right one more often than people expect. Adults generally need seven to nine hours, and quality counts as much as quantity — a full night broken into pieces does not restore you the way an unbroken one does. Two weeks of a consistent bedtime is a cheap and surprisingly decisive experiment.
2. Sleep apnoea
Loud snoring, gasping at night, morning headaches and daytime sleepiness are the pattern. It is common, it is treatable, and it is very often missed. If a partner has mentioned your breathing at night, that is worth a doctor's appointment.
3. Iron, B12 or vitamin D
All three are common deficiencies, all three cause fatigue, and all three are answered by a simple blood test rather than speculation. Guessing at supplements without testing is how people spend money on the wrong bottle for a year.
4. Thyroid function
An underactive thyroid produces fatigue alongside weight change, feeling cold and low mood. It is one more line on the same blood form, and it is easily treated once identified.
5. Mood, stress and the load you are carrying
Depression and sustained anxiety are physically exhausting, and the tiredness often arrives before anyone recognises the mood component. If rest genuinely does not help, this belongs on the list rather than at the bottom of it.
6. Alcohol, caffeine and their timing
Alcohol reliably fragments the second half of the night even when it helps you fall asleep. Caffeine has a long tail — an afternoon coffee is still in your system at bedtime for many people. Both are easy to test by changing the timing for a fortnight.
What to do
Start with sleep, alcohol and caffeine, because they cost nothing to change. Then ask for a blood panel covering iron, B12, vitamin D and thyroid. Fatigue that is sudden, severe, or comes with weight loss, fever, breathlessness or night sweats should be seen promptly rather than worked through slowly.
The reassuring part is how often the answer turns out to be one of the first three.