Headaches

I'm trying the weighing and I don't even get headaches!

You mean it's not the natural thing to do? Might explain my wife's raised eyebrows, I suppose.

Before you discount the dehydration option (as many underestinate their losses) weigh yourself (naked and after emptying your bladder) pre-ride and then do the same on return (once again as nature intended and with an empty bladder). I've frequently been surprised by the difference, especiall when you convert it to %age loss.

Maybe, like DaSy says it could be a sugar spike thing? I get dizzy spells after eating sugary foods and have to lie down for a while to let it pass (according to doctors this means I am healthy!)

I get the same, and think it is a nutrition issue. I've found that avoiding the things I really fancy when I get back helps. By that I mean anything sugary; cakes, biscuits and sweets etc and definitely espresso based coffee (my favourite!).

I always want a double espresso shortly after I come back, and always end up with a major headache for the rest of the day.

I now have something like sardines on toast and orange juice, and avoid sugar and caffeine for a couple of hours after a long ride. This has made quite a difference and the headaches are much rarer now.

Could it be your sinuses or perhaps it's something to do with you position on the bike that is so slight it doesn't affect you over shorter distances?