What does the server write when nobody is logged in?

I've noticed that when nobody is logged in, the region files are still constantly updated. Does anyone know what the server is doing to them, and whether this might be able to be suppressed somehow?

It's very annoying, because it my server doesn't have people in much of the time, and it's constantly inflating the size of my backups. With the new region format, these files are quite large, too.