system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: When going to bed after a day at the waterpark, doing rollercoaster or at sea, I can feel the "movements" of the day. Why is that?

Also, why does it only happens with some activities (ie: i don't feel myself walking) ?
gpt
Your inner ear detects your movements and send signals to your brain so you can respond to the situation and balance. Your eyes are also sending similar information. Eventually your brain gets used to the same signals being sent at regular intervals (waves being regular at the beach). Your brain starts to expect this movement as normal, and keeps compensating. When you return to land, your brain is still compensating for motion that isn't there.  There's lots of other stimulus for the rest of your day at the beach so you might not notice, but at the end of the day when you are in a quiet, dark room with little stimuli you'll feel that compensation again, and feel like you're still rocking. 

You can override the signal by giving your brain something else to base stability on (same thing for when you have the dizzies after a few too many drinks). You can put your hand or foot on a flat surface and your brain will recognize that you aren't actually moving. Anecdotally, I've found that touching the floor or a table works better than a wall when lying down.