I've sold lots of overnight spots on the basis that while there are few listening, they're almost all working (making money) and are not hearing about all the great sales and businesses wanting everyone's business. I noted overnight workers were not listening during the day. I pointed out that with a small but loyal group of people, you and not your competitor, were telling them about the great offerings of the business, showing them that to your business, these listeners mattered. I noted a smaller commercial load meant their overnight spots were better retained by the listener.
Granted, I was selling in an unrated market and the price per overnight spot was $1 versus $12-$16 during the day. We were able to load up 6 to 8 units an hour with the $1 per spot add on to an existing schedule for 12M to 6 am. A client buying 3 spots a day 6a-7p with 1 per daypart ran $42. We'd say for $2 or $3 more you can reach those you miss in the 6a-7p time frame.
Sure, it didn't pay the live jock and electric, but $6-$8 an hour sure beats $0.
I contend we must have more listening at night than we think or folks like Art Bell would not be so commonly known.