At night the AM stations you can hear (clearly or otherwise) changes radically. many laymen actually have a fair understanding, many do now. I'll glaze over it here before I get to the arcana. After dark solar radiation is reduced and ionization changes occurr in a part of the upper atmosphere we call the ionosphere. This change reflects AM radio signals back toward earth. But there is an angle of difraction, like bouncing a beam of light off of your watch onto the wall. Because of this, a station can be heard hundreds of miles away. In some cases even thousands. this part of the Am radio signal is refered to as the skywave.
But without massive signal management, this would oversaturate the AM band coast to coast rendering it completely useless at night. In order to reduce this interference the FCC has required certain staitons to power off or power down at night. Some of them power down but then use directional antennae to maximize their coverage over major cities, and/or minimize interference to primary stations and adjacent channels. Many staitons actually broadcast from different completely antenna arrays at night to accomplish this.
None of that is a huge surprise. Here's the surprise: some AM stations actually power up at night. Kind of counter-intuitive isnt it?
There are not many of these. But a few years ago the FCC began allowing non-standard power levels in certain cases. The general idea is that night time channel protections are different on certain frequencies. Therefore more power should be permitted in those cases than if a daytime allocation exists locally to the station. Another reason is that a station may prefer to have an omni directional day signal of lower power than a directional array of higher power.
620 WSNR-AM - Newark, NJ - day: 3kW, night: 7.6 kW
950 WWJ-AM - Detroit, MI - day: 12kW, night: 50 kW
950 WNTD-AM - Chicago, IL - day: 1kW, night: 5 kW
1330 KJLL-AM - Tucson, AZ - day: 2kW, night: 5 kW
of course some stations ignore the powerdown rules and operate illegally at night just to reach a larger area. Some do this out of laziness or incompetence. But some stations deliberately violate their license. They kill the carrier signal at sun down then just power right back up again. Or they to a pattern just looser than the FCC would permit. They usually get caught.