"Your compass spins wildly!" In one of the earliest text-adventure games that I played on a primitive computer many decades ago I remember a room in which one couldn't tell direction. The same thing happens to me on Sunday 29 August, perhaps coincidentally 24 hours after an all-night trail run. Fortunately I recognize the symptoms: I had it briefly three years ago, on 2007-03-10. Then and now, when I roll over in bed I suddenly feel dizzy. The room rotates around me for several seconds, then settles down. When I change the orientation of my head again, it happens again. I recognize it, and a few days later my helpful and supportive doctor (Deena Shapiro) confirms the self-diagnosis.
It's Benign Paroxysmal Positional Vertigo a.k.a. BPPV, and is caused by out-of-place calcium crystals in the inner ear, called otoconia or otoliths ("ear rocks"). They move around when the head turns, disturb the fluid in the motion-sensing semicircular canals, and cause a feeling that the head is turning when it's not. The eyes twitch (nystagmus) and the head swims. My BPPV hasn't gone away for a fortnight and thus far isn't too much of a problem. It strikes when I try to look straight up at the stars, when I lie down or sit up abruptly, and when I tip my head sideways to read the titles of books on the shelf. After a few seconds the merry-go-round stops. We'll see how long it persists; the doctor recommends living with it if it doesn't get any worse. So far, so good: it's a cheap thrill, provides considerable amusement to colleagues when I demonstrate it, and doesn't seem to interfere with my running or other activities. And I haven't fallen down yet!