When I first came to Japan, I could read and write kanji as I’d studied Chinese (just needed to learn the Japanese pronunciation) but not the two phonetic alphabets, hiragana and katakana. So I set about learning those. My wife told me that, one night, I sat bolt upright and declared, “Let’s eat okonomiyaki!” then fell back to sleep. I have no memory of this, but I suppose my brain was processing what I’d been studying - おこのみやき - in the form of a dream. The Guardian has an interesting article on theories about why we dream.