Why do we feel as though we're rocking after getting off a boat?

08 May 2011



Hi, I'm from Venezuela. Why is it that when I go to the beach and spend some time in the water, the same night just when I'm falling asleep I feel the movement of the waves around me?


Chris - It's very common, it turns out. The records go back to the 1700s. Erasmus Darwin, who was Charles Darwin's grandfather, actually recorded one of the first, if not the first, description of this condition. And what Darwin wrote was:

"Those who have been upon the water in a boat or ship so long that they have acquired the necessary habits of motion of that unstable element, at their return on land, frequently think in their reveries or between sleeping and waking, that they observe the room they sit in, or some of its furniture to librate like the motion of the vessel. This I have experienced myself and have been told that after very long voyages it is sometime before these ideas entirely vanish. The same is observable to a lesser degree after having travelled some days in a stage coach, and particularly when we lie down in bed and compose ourselves to sleep."

He wrote that in 1796, it's reassuring to know that people are still having the same problem today. This actually is a medical condition. The fancy name for it is 'mal de debarquement', in other words 'illness of disembarkation', or feeling unwell when you get off the boat. For some bizarre reason, it appears that the incidence is highest in women in their 40s. It may just be that women in their 40s are more prone to going on cruises than people of other demographics, I don't know! But it's amongst them that you tend to get most of the medical reports. Luckily, it's generally a temporary thing. What scientists think is going on is that you have in your head a model of the world and how you are relating to it. In other words, if the world is moving, then you're modelling that movement and working out how to compensate for it either with movements of your head and eyes or your balance system so that you don't fall over. When you're on a ship, because of the constant movements, your brain has to 'de-tune', or damp down, that response a little bit. If it didn't, you would continuously be over-correcting for it, which might underlie why you get seasick in the first place, and why after a period of time at sea you stop feeling seasick. So, when you then come back onto land, the signals are being fed into this system which models how you're interacting with the world. When those signals are coming in now, you're not continuously in motion so the very thing that was expecting you to be in motion is no longer always seeing motion. As a result the model is predicting how you should respond to the movements around you incorrectly. So you experience these rather strange sensations as though the world is continuously moving. And it takes a little while to 'un-learn' this newly learned behaviour and to revise your model so you then don't keep jumping around all over the place.


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