"Oh, heh, cute that scientists need to confirm what's clearly obvious to me." Often with an anecdote about how they've always known it, and with an explanation about how it works (obviously) which you think would be unnecessary if it was such common knowledge.
When we realize it's not common knowledge, we get excited and/or become proud of ourselves, and explain what we think to indulge ourselves (because we were just given evidence that the knowledge isn't as common as we thought).
It is pretty common to explain sleep as when your mind cleans up and store stuff in long term memory. From there it is largely a point of view distinction between “keeping stuff you want to save” and “throwing away stuff you don’t want to save”