I have had different types of brain, and while im sure the answer you get will vary, this is at least what seems to work in my experience.
If your brain has a skeleton that would lift it off the sand, you can put it in the sand. If your brain is one of the types where its 'skin' plops over its sides to such an extent that it would hit the sand, then I would advise putting it in a rock.
I have literally had a very happy brain, that I moved from a rock to sand JUST while cleaning, and it closed up and would NOT come back out til I put it back on the rock.