I started in radio at a sunrise to sunset operation. Still did well, because in the 60's, people understood that some stations were not on all the time.
Next stop was a daytimer that came on at 500 watts every day at 6am. Increased to 1kw at sunrise. Again, no revenue problem. We could have done better at Christmas time if we had been on til 7pm, but thems the breaks.
In the early 90's, I was the OM at a daytimer. 500 watts 6am til sunrise, 1kw day, reduction preceeded after sunset down to 42 watts. Still covered most of the market. Baseball, NFL and high school football sold well. The owner fought me hard against 24/7 operation, but I finally prevailed. My arguement is this. These are certainly not the 1960's. Then, there were FM and TV stations even, that were not 24/7. Some FM stations, even in major markets, did not sign-on til noon (WSID-FM Baltimore a prime example). People understood this. Now, everything is 24/7. All FM and TV. Throw in 300+ cable channels. If you are an AMer, you are at a disadvantage to begin with. If you are here one hour and gone the next, no one takes you seriously. If one of your 50+ year old listeners does go out after 7pm, turns on the radio, and you are not there, they will punch up another station. The next morning, when they get in the car, its THAT station which will come up when they turn the radio on. Not yours!
Even back in the 60's, I knew of adertisors who would not place ads on daytime-only stations. They didn't want to advertise at night, they just thought it was a waste of money to place spots on a daytimer. 50 years later, few are going to take you seriously.