Yes, almost certainly in this case Google sees traffic die off when a data center is in the dark. Specifically, there is a diurnal cycle of traffic, and Google usually routes users to close-by resources. So, late at night, all those backends which were running hot doing low-latency replies to users in near-real-time can instead switch over to processing batches. When I built an idle cycle harvester at google, I thought most of hte free cycles would come from low-usage periods, but it turned out that some clusters were just massively underutilized and had free resources all 24 hours.