[Avodah] Why is the minhag to say selichos right after chatzos on the first night?

[Marty Bluke]

The prevalent minhag has become to say selichos right after chatzos on
the first night of selichos. What is the makor for this? In shulchan
aruch it says that the minhag is to get up early to say selichos. I
didn't see anywhere a distinction between the first night and the
other days. It is clear from the shulchan aruch and mefarshav that the
best time to say selichos is a little before alos hashachar.