Low to mid frequencies, especially AM broadcasts in the mediumwave and shortwave bands (and to some extent longwave), travel most efficiently this way at night, as they act more like the earth's natural electrical field than other higher frequencies (which act more like light), or lower ones (which act like magnetism). Because the solar wind "blows" the ionosphere toward the earth on the day side, and away from it on the night side, this natural radio "mirror" is much higher above the surface at night -- allowing skywave propagation to reflect much further.
During the daytime, mediumwave and shortwave transmissions travel mostly by groundwave.