Why am I so tired?

Question:

Why am I so tired?

Right, I know more than one person has asked this before (try hundreds, in fact...), but I thought it best to give specifics.
Eighteen monrths ago I was the fittest I've ever been (still not that fit in objective terms, but not bad for the UK), was doing well in the TA, had just joined the Coastguard (as a full-timer), was working toward my pilot's licence and generally was pretty chuffed with life.
Now, however, my fitness has gone to pot (I can barely even walk the mile to the gym without being exhausted, let alone do anything when I get there) and I just can't be bothered going to the barracks in the evening or going out with my friends. I haven't flown in months. I can't be bothered doing things I really want to do, if you see what I mean? My head hurts constantly, I'm in a more or less permanently foul mood and it feels like I'm watching the world on TV rather than living in it. It's like something important has broken or gone missing.
Any ideas, O internet peeps?

Additional Details

1 week ago
Why is this in this category when I chose 'health'...? Any thoughts?

1 week ago
I'm beginning to wonder if the fact that I'm now working shifts (which has knackered my eating habits completely) might have something to do with it. A lack of direction in life might also be contributing. Saving lives is all well and good, but I don't get the fun, exciting side... Meditate on this, I shall. Keep 'em coming.

Oh, and Oldpinky: there's no such thing.

Best Answer - Chosen by Asker: 1 week ago
Why is this in this category when I chose 'health'...? Any thoughts?