http://en.wikipedia.org/wiki/Mal_de_debarquement
I’m surprised at the rarity of this condition, but maybe that’s because I get it myself, and assumed everyone else did too.
My first experience with MdDS was after taking two boat trips and two long-haul overnight trains (on the top bunk, where the sway is most noticeable). I spent the next week with that teeter-totter sensation of a ship in moderate swells. Even when standing still, or lying down. Not unpleasant, but certainly weird.
Mal de debarquement syndrome (MdDS, or disembarkment syndrome) is a rare condition usually occurring after a cruise, aircraft flight, or other sustained motion event.
Symptoms most frequently reported include a persistent sensation of motion usually described as rocking, swaying, or bobbing; difficulty maintaining balance; extreme fatigue; and difficulty concentrating (“brain fog”). Other common symptoms include dizziness, headaches and/or migraine headaches, confusion, and anxiety. Many patients also describe ear symptoms such as hyperacusis, tinnitus, “fullness”, pain, or even decreased hearing. Cognitive impairment (“brain fog”) includes an inability to recall words, short term memory loss, and an inability to multi-task.