I emailed the ed about this last week, It has been happening for months and I kept forgetting to mention it. There seems to be a problem just with AB and I think it is something to do with the greenies changing to grey possibly and the system does i kind of defrag maybe.
It's probably the time of night that database backups and cleaning occur. Or possibly even search index updates. All of these can cause a web site to go a little awry.