Twice in the past ten years or so, I have passed out while using the rest-room after abandoning a hard brevet. Both times were after the first 400km of a multi day ride that was much hotter than I expected.
I did some research on this to see if I could prevent it from happening again. I found this page to be very useful https://www.health.harvard.edu/heart-health/when-the-lights-suddenly-go-out.
I had suspected this was related to dehydration. When you stand up quickly, especially from a deep sleep, the blood pressure in your brain drops suddenly and the heart takes a while to catch up. If you are dehydrated, your blood pressure is low to start with.
In addition, if you have recently eaten - and this is common at the end of a 400k, your digestive system has more than its usual allocation of blood, making less available to the brain.
So the combination of exhaustion, suddenly standing up, dehydration, and a heavy meal all add up to a recipe for passing out. It's amazing it hasn't happened to me more often. So what can I do about it?
Normally there are warning signs. I feel dizzy or break out into a clammy sweat a few seconds before passing out. I could sit down and put my head between my knees (apparently this actually works). It also seems crossing your legs and squeezing your leg muscles helps pump blood up to your head.
I should also make a more concerted effort to re-hydrate after a ride and maybe try not to eat too much before going to bed.
Last time I went down I cracked my head and elbow pretty badly. I don't want to do that again.
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