Last night I went for a 2 hour ride. Pretty tough ride. today I have a splitting headache and feel very lethargic. I am pretty fit and ride around twice a week with a 20 mile commute 3 times a week. This has happenned a few times before. ANy ideas?

Dehydration perhaps ?

After night rides I struggle to get a good nights sleep and quite often feel a bit rough the next day, not saying this is to do with your issue.

I am sure somebody more knowledgeable will jump in, but to my layman's medical understanding, that sounds an awful lot like dehydration and not replacing electrolytes as well as fluid. Normally what it is with me when I feel like that.

Get drinking!

Kev

I think you are right big scot nanny. Get drinking and take some salt.

+1 for dehydration.

I feel the same if I do a fairly hefty ride after a day at work where I haven't concentrated on my hydration. I work in an office where I get dehydrated far more than I do anywhere else. Couple that with lots of coffee and I'm peeing dark yellow late in the afternoon. If I'm riding in the evening, I generally (though not always) tend to be aware of my hydration levels and I'm already on the energy drink an hour before leaving work.

Remember that hydration is a slow process and, having become dehydrated, it takes a long while for the body to get fluid to all your cells.