There was a time when stations were not so overloaded with debt that the entire overnight Midnight-6am had only one or two sponsors, if that. I worked overnight in a large market large station that had no commercials overnight.
The commercials overnight have nothing to do with debt but the idea that the station can offer unsold inventory at a huge discount and have some cash rolling in or that they are playing a few freebies to match what the ad agency will pay per unit (ie: ad agency says $20 but your rate card is $25, so you add free night spots to hit $20 a spot).
I knew many, many stations in major markets that had mostly zero commercials midnight to 6. Then again back in that time there were fewer stations. I have worked a station in a college town that had about 6 to 8 units an hour after midnight (and all those advertisers were open all night). If I had to be stuck on overnights, that was one that seemed way more midday than middle of the night (the request line was pretty busy at 3 and 4 am).