For the right definition of "major". Search, Ads, GMail, Docs, absolutely. If you only have 50 million DAU then you may not even have an SRE let alone one in multiple time-zones. Most teams have an on-call rotation and you get a phone-call an hour after you fall asleep unless you recently did a push or go to bed early. (Or you just forget all the pages that happened during normal hours and only remember the annoying ones.)