One leads to the other, and the line may be fine. To paraphrase Justice Stewart, I know the difference when I experience it. Feeling tired is quite different than the disoriented malaise of bonking. The former is, for me, mostly a physical sensation, but the latter is physical and mental. In the early stages of bonking, I may still be aware enough to recognize that I'm losing my mental sharpness, that I'm not thinking clearly, or not really noticing the terrain or other conditions of the ride. But even when the bonk first starts to hit, I can still sense that something is wrong, though I may not quite understand it at the moment, in my disoriented state. Sometimes, a sense of being on "auto-pilot"--as if I'm not really seeing anything, even subconsciously--can be a clue that I'm beginning to bonk.