Speaking for the assembly lines in the US both @ GM and Ford plants which I've been involved in contractor work, they usually don't work around the clock to allow time for minor maintenance, working extra time before/after the shift, parts resupply to the line etc. For years, I'd have a crew in during those off hrs in order to slip in and make modifications to equipment so as to pull ahead for any changes in the system. Usually a model change would be released around September +/-, we would often be working on this change the preceding Christmas time, any major holiday, weekends, basically anytime there was available time to swap out/modify/install new or replacement equipment.