Critical Hours is the time from sunrise to two hours after sunrise, and from two hours before sunset until sunset, local time. 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.
Canadian Restricted Hours are similar to Critical Hours, except that the restriction results from the January 17, 1984, U.S.-Canadian AM Agreement. Canadian Restricted Hours are called "Critical Hours" in the U.S.-Canadian Agreement, but in the AM Engineering database, the FCC calls them "Canadian Restricted Hours" to distinguish them from the domestically defined critical hours. Canadian Restricted Hours is that time from sunrise to one and one-half hours after sunrise, and from one and one-half hours before sunset until sunset, local time. U.S. stations operate with restricted hours because of Canadian stations, and vice-versa.
Those radio stations that are required to 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 with nearby frequencies.