This refers to a Japanese proverb or superstition that reads, 食べてすぐに寝ると牛になる (Tabete sugu ni neru to ushi ni naru), which literally means that if you sleep right after the (or a) meal, you will become a cow. It has been speculated because it is considered impolite or not good for digestion.
~Jumping~~Gasp~Huh? Where am I?~Rumbling~Nya!?Rumbling and Swinging~Rumbling~~Rumbling~We're heading to the market...~Rumbling~~Rumbling~So you'll have to say goodbye soon.What's wrong?I heard spicy food can cause weird dreams, but the ramen wasn't that bad!~Rumbling~