Headcanon Danny actually does sing Arin Sleeptime Junction so he can fall asleep and when the others find out they demand a sleepover so he can sing it to them, then drag him into the middle of the cuddle pile as thanks.
Sorry I thought I’d replied to this. Things have been so hectic over the last couple weeks.
I find this so cute and think at first it would happen whenever Arin was stressed and Dan knew he needed to chill out, so Dan would sing that song to him, mainly to make Arin laugh but he then falls asleep and as time goes on it just becomes a thing. One time Ross sees them and records it, he wasn’t even planning on making fun he just found it cute. Then when he returned to the main part of the office everyone was asking why he’s smiling so he had to show them and that led to a sleepover with Dan singing them all to sleep while cuddling. (They all fought over who got to be cuddled up to Dan)