the night before 与 last night

题目

the night before 与 last night
下面句子是高中必修4第一单元第一段,请问最后一句话中为什么用the night before,为什么不用last night?印象中是间接引语用the night before,直接引语才用last night啊.
It is 5:45 am and the sun is just rising over Gombe National Park in East Africa.Following Jane's way of studying chimps,our group are all going to visit them in the forest.Jane has studied these families of chimps for many years and helped people understand how much they behave like humans.Watching a family of chimps wake up is our first activity of the day.This means going back to the place where we left the family sleeping in a tree the night before.

答案

因为Last night 就是指 “昨天晚上”的意思,而the night before 是指“前一天晚上”.文中指的不是说话时刻的前夜,而是Left the family的前夜.所以不是昨天晚上,不用last night.