관계대명사 which의 선행사는 무엇인가요?

They need to be involved in those activities again in order to fall asleep the next time they get sleepy, [that/which] means, when they get tired but those sleep associations are absent, they start to cry.
여기서는 which가 답인 것 같은데, 이때 which의 선행사는 무엇인가요?