I'd love to know about any station that sells anything at all in overnights. Most stations bill $0 in that period, so barter is an option. However, the principal bartered overnight shows take some daytime inventory for the deal.
And few stations bill significantly after 7 PM. Those are mostly bonus spots you hear if you hear any at all.
I'm guessing that your radio background does not include any sales experience.
There was a time when overnights were considered a lead-in for morning drive, and the last couple of hours may have had enough income to pay an overnight jock and still make a profit. Of course, that was a long time ago when there were recognizable overnight personalities and sponsors that pursued their audiences. You've probably heard of Alison Steele. Locally there were a handful of overnight people who had a handful of steady sponsors.
The evening show is more likely to run 6-10 PM these days. Locally, at least a couple of stations are live and are running enough commercials to be in the black.
There are times when a spreadsheet doesn't count all of the profit. If the money is equal - or even close - keeping the money in the local economy and the goodwill of advertisers and listeners provides a lot more benefit than a spreadsheet accounts for. The corporate radio idea that a "national talent" deigns to bless us with their "quality broadcast" is overblown. From a corporate standpoint, paying somebody with a fat contract to localize some liners so they can syndicate the show looks like a deal. From a local standpoint, giving up avails and/or cluttering stopsets with additional inventory may not make it such a great deal.
I look at radio from a much bigger perspective than "just sales" or "just programming" or "just management". You need to look at it all holistically - including the community you're in - to get a real picture of what's in the best interest of the long-term viability of the enterprise.