As I understand it, the most likely cause of tired all the time is lack of sleep. Easy to check: throw away your alarm clock. If you're habitually using a machine to shorten your sleep then you'll be tired, and getting rid should fix that. Apparently 'stress' is another common cause, but I don't know what 'stress' is. It just appears to be a word for 'bad stuff possibly including diseases and definitely including things that spoil your sleep'.

After those two are ruled out, then you should go and see your doctor. Fatigue isn't normal, and there are about 200 different diseases that can cause it. Go pre-armed with arguments like 'I get twelve hours sleep every night and I still need an afternoon nap' and 'I've lost the ability to read, I can't concentrate', then she'll take you seriously.

She should check for anaemia and thyroid function, plus loads of other things. If after having tested for anything she, you, the NICE guidlines and the internet in general and any medical friends can think of, you're still inexplicably tired but apparently perfectly well, then you're probably on the verge of a CFS diagnosis.

I think up until that point, medicine and I are in agreement. Then the craziness starts:

Apparently low iron can be 'low for you' rather than 'out of the population normal range'. If your iron levels are 'low normal', then there's a nice-tasting iron supplement that you can try called Floradix that gets good reviews on Amazon. Don't go overboard though, high iron levels are also a cause of fatigue and other nastiness.

Also try making a list of symptoms, by which I mean 'everything about your state that isn't the same as it was the last time you remembered being unambiguously bouncy and enthusiastic'. A lot of it just looks like aging. Then work out what your score is according to Billewicz's test.

If Billewicz would have thought you were hypothyroid, then it might be worth trying to find a doctor who is prepared to give you a trial of thyroid hormones. But be very very careful. No one knows anything about this except hunches, and any doctor prepared to do this is pretty much by definition acting against best practice.

On no account mess about with this without medical advice. These hormones are very powerful and can be very dangerous.