The process is called skywave propagation. AM broadcasting utilizes long wavelengths to get sounds to your radio. These wavelengths bounce off of the ionosphere (electrically charged part of the atmosphere) when they're broadcast. During the day, the sun's rays push the ionosphere closer to the Earth's surface which means AM waves can only propagate so far before reflecting back During the night, however, the rays aren't pushing the ionosphere so it rises farther away from the Earth. Thus, at night, AM waves travel farther up and then can reflect over a greater distance.
