I'd really caution against leaving any sodium hyrdoxide based product (is that what starsan is) in the lines overnight. I'd limit to no more than 2 hours.
I've been part of a research project for using multiple enzymes in breweries, one application is beer lines. Beerlines like many cold dark sugar rich environments are havens for biofilm. Biofilm form a primary attach point for bacteria and protein, hence the removal in the circuit increases cleaning efficacy.
Sadly, the caustic cleaners do not remove biofilm sufficiently (biofilm actually react but toughning thier outer layer and become more resistant to removal), hence you may remove some of the biofilm and what is clinging to it, but not the majority.
So, the caustics do a job, but not a very good one.
Scotty