I've had the same problem for 25 years. The fitter I am the worse it is. Cooling down doesn't make a difference. It can happen any time that I am lying down or sitting still and get up quickly. It's worse for the rest of the day after a long weekend ride. Perhaps because I am more relaxed than I am at work.

I deal with it by not springing to my feet so fast. I am normally very energetic and will jump to my feet to go do something. But if I do that when I have been sitting still and relaxing, I'll partially black out- my vision goes black and there's a noise in my head. If I sit up, wait a few seconds, then stand up like a normal 50 year old there's no problem.

I've talked to doctors about it. There's a name for the syndrome but I have forgotten it. The problem is that the valves in your legs are open and your blood pressure is low, so when you stand up there's not enough blood pressure to the brain. There are some drugs that have some effect but they're not really worth taking unless it's really a problem for you. I deal with it by getting up slower and waiting it out when I forget. As long as I find something to hold on to so I can keep my frame of reference, having blacked out vision isn't so bad. It doesn't last long.

When I asked my doctor recently he told me about a trick- right as you get up, grip the fingers of each hand together and pull. You don't have to pull hard. That supposedly activates the same system that closes the valves in the legs and raises the blood pressure. It may work: I keep forgetting about it until after I've gotten up.