(This, anyway, is the kind of tiredness that happens after concentrating on the same activity without sleeping for, say, 19 hours: e.g. driving, grading, studying.) Edit: this actually seems a very apt metaphor to the increasing difference in my systolic and diastolic blood pressure when I need to sleep: the blood goes out with normal rapidity, but doesn't want to return. My body loses the ability to calibrate the two (already less calibrated than normal) until sleep resets it. Something Big Science has yet to explain, but which seems natural enough to me.
feels like the single rhythmic strain of my body diverging into two separate streams: one speeding further and further up, another slowing further and further down--until the space between them is dizzyingly large, and I can no longer maintain a steady pace, and fall down.