Why is it a "bell" in the morning and a "drum" at night?

According to Buddhist regulations, drums are played in temples at night and bells are rung in the morning. It is a metaphor for awakening people's voices or words, and describes the delay of time.

Source: Southern and Northern Dynasties, Geng Xin's "A History of Riding Nanshan and Yuwen Moon": "The garrison building rings the night drum, and the mountain temple rings the morning bell."