During this time, certain American radio stations may be operating with reduced power as a result of Section 73.187 of the Federal Communications Commission's rules.
[1] Canadian restricted hours are similar to critical hours, except that the restriction results from the January 17, 1984, U.S.-Canadian AM Agreement.
[1] Those radio stations that must lower their power during the critical hours are required to do so because this is when the propagation of radio waves changes from groundwave to skywave (at sunset) or vice versa (at sunrise).
This can cause radio stations to be picked up much farther away, possibly causing interference with other stations on the same frequency or adjacent frequencies.
This is especially important in autumn and winter, when these stations might otherwise need to be off the air during the important morning and afternoon drive times, when AM radio listening is at its highest.