This is actually something that's been bothering me for a long time and I can't really find anything about it. It's well known that Japanese lacks relative pronouns, as such “寝ている人”, “寝ているベッド”, “寝ている時間” and “寝ている理由” all have widely different interpretations based on what makes sense despite having identical surface-level grammar.
In practice, one can use other nouns to shift the interpretation such as “ゲームする人” and “ゲームする相手” generally having different interpreations but with specifying specific locations I'm honestly at a loss. If one really would want to somehow set apart the bed under which something is sleeping, opposed to the bed in which something is sleeping, how would one do that? I would assume that something such as “下で寝ているベッド” would be used, but I've also never seen it.
by muffinsballhair