The vast majority of AM radio stations are the local type that either go off the air or cut their power at night to 5000 watts or less. Only a few maintain or increase their power at night to 50000 watts.
      There are two basic categories of 50000 nighttime AM stations: regional and "clear channel". The regional stations can only broadcast their high power signal in certain directions. If you happen to be located in the pattern the directional station broadcasts and there is no local station on that frequency you should be able to pick it up. The clear channel stations are allowed to broadcast their signal in all directions. This allows them to cover a much larger area. Although the clear channel frequencies are no longer truely clear channel because local stations can broadcast on the same frequency, if you are within 750 miles and there is no local station on it's frequency, you should be able to receive a clear channel station's signal. With a good antenna and clear weather you may be able to receive signals much farther away than that.