Nevertheless, she is always in motion. On the calmest day, early in the morning when the water in the marina is a sheet of glass, if you watch that hanging kerosene lamp over the dinette table, you'll see it swinging, ever so slightly.
When you live aboard, that motion gets built into your world view - it quite literally disappears from your conscious perception. But it is still there, in some more primitive part of the brain, a part which communicates upward only poorly, and then only by coloring your consciousness with vague feelings.
When I sleep ashore, for example, I find it difficult to fall asleep. There is something vaguely wrong... for lack of a better explanation, the bed feels "dead". It takes several days for this discomfort to fade.
But it takes no time at all to re-accommodate when coming back aboard.