The Midnight Effect - Who-How?

Radio stations have to reduce transmitting power at the evening per FCC regulation.  It is Section 73.187 of the Federal Communications Commission's rules to reduce power from 2 hours before local sunset to 2 hours after local sunrise.

@cleeds   This is from FCC website:

"Most AM radio stations are required by the FCC's rules to reduce their power or cease operating at night in order to avoid interference to other AM stations.  FCC rules governing the daytime and nighttime operation of AM radio stations are a consequence of the laws of physics."

@cleeds  Thank you for the info.  Do I understand correctly, that some AM stations have to operate at lower power at night (and critical hours), while "some" means stations that operate at frequencies of other (older?) stations?

Electromagnetic radiation or ambient electrical noise may play some role, but we are also getting quiet at the evening.  Our internal noise is reduced and focus increased (and the booze of course).

@cleeds   They have to operate at reduced power,  but I misunderstood definition of critical hours.  Critical hours is 2  hours before sunset to sunset and sunrise to 2 hours after sunrise.  It has something to do with greatly increased propagation of the signals during critical hours ("skywave propagation").  I don't understand if critical hours are in addition to reduced power at night or power has to be reduced only during critical hours.  In previous quote from FCC they mentioned "reduced power at night" (inconsistent).   They mention daytime and nighttime power.  Do they require nighttime power in addition at "critical hours".   Few years ago I was listening to interesting AM program and always around 6-7PM signal was dropping rapidly resulting in a lot of noise.

"For AM broadcast stations, the term critical hours refers to the time periods of sunrise to two hours after sunrise, and two hours before sunset to sunset.  During these periods, the ionosphere has commenced its transition from daytime to nighttime conditions (or vice versa), resulting in greater coverage than would be expected from a daytime-only analysis.  But because the transmitting station operates with its daytime power between sunrise and sunset, the extended skywave signal can be strong enough to interfere with other stations.  This daytime skywave phenomenon was the focus of a protracted rulemaking proceeding that commenced in 1947 and terminated in 1959 (see Docket 8333 for a limited number of decisions in this docket) with the adoption of Section 73.187, Limitation on daytime radiation (also called the "critical hours" rule), and Figures 9, 10, and 11 in Section 73.190.  The rule provides for operation at a lower power during critical hours time periods to limit interference from new or changed Class B or D stations (where the changes were made after 1959) operating on frequencies specified in Section 73.25, to Class A AM stations on the same frequency."