Originally Posted by banerjek

Correct. But that logic can be extended to just about any time period exceeding 30 seconds.

While you can certainly push yourself to the point that you can barely stand after a shorter ride or an interval set, you'll never feel sleepy. But this is a common situation for me after longer rides. That I wake up feeling great after just 10 minutes in most cases tells me that it has more to do with what's going on chemically in my body than fatigue and that whatever is going on is not just a function of effort since I push myself harder on shorter rides. Wrapping my mind around what is going on was the motivation for the original question.