Yeah. Runners call it the wall; cyclists call it "bonking".

I've been told that it can come on more suddenly and acutely for cyclists. As a marathoner I find that I feel it coming for several miles, which is tens of minutes. The activities are somewhat different so it's not impossible that they manifest differently.

But the basic idea is similar. You've got so much readily available glycogen in the body, and when it runs out, you have to shift to a different metabolic pathway -- and your body starts sending up warnings that you are rapidly running out of reserves and must stop now.

It's as much mental as physical at that point.