In Korea, there's a popular theory that says that: "If you eat a quarter of an Iceberg lettuce, you will fall asleep". Thus, amongst truck drivers in Korea, lettuce is known as something that should not be eaten before work.
俗説で「玉レタスを4分の1程度食べると眠くなる」と言われ、韓国では仕事前のドライバーが食べてはいけない食べ物として知られている。