The Midnight Effect - Who-How?

I heard it on Friday/Saturday morning and it was almost like someone had flicked a switch. Or more like someone had suddenly placed extensive acoustic treatments in the room.. the soundstage just opened up in all directions and the sense of presence and of natural tone….

It sounds as if the acid must have just kicked in.

I "warm up" my ears before seriously listening. I start at very low volume while "getting my head together". I allow my brain to "tune in" to whatever is on at that volume until I can hear details and depth. At some point the "switch gets flipped" and I turn up the volume. I'm sure there are various electrical and electronic variables changing, but my focus is already fixed on the sound of the music. 

Our electrical grid is an amazing thing where your power can be coming from a solar panel this morning, a wind farm this afternoon, a fossil fuel plant this evening, and a hydro station overnight - with none of them being anywhere near you. On its way from the source to your speakers, that power goes a long way, through all manner of equipment, and past all sorts of sources of interference.

Every time the utility changes the source of your power, there will be a corresponding change in the "hash" that is induced on the power lines and comes along with it to your doorstep. It might be radio frequency noise coupled from a nearby radio station (near to a power line 100 miles from you) or an ultra low frequency created by the fluctuations of a 345KV line losing a varying amount of energy as it slowly swings in the breeze on a foggy evening and some power bleeds off as static energy.  

As an electrical engineer who has done load flow analysis for utility companies, I have worked on power systems where they had thunderstorms in the power plant switchyard when the salt fog rolled in off the ocean, and the same system needed to put out 370KV at the plant to get 345KV out at the far end of the line. Most of the rest bled off into the air although some was lost to resistance heating and other equipment losses. What shows up at your door constantly goes through all kinds of waveform shaping, matching and combining of multiple power generation sources, weather conditions, temperatures, and equipment load variations - some planned, and some not. 

Where I live, there are two predictable utility changes that happen every day. One is when the water company switches from one pumping station to another about 1:30 in the morning.  For about 2 months some years back, there was enough difference in the water pressure to create a water hammer pulse in my area that was strong enough to trip the pressure alarm in my building's fire suppression system and we had 30+ families rousted out of bed until the fire department confirmed the true source of the call-in. There were a lot of really ticked-off people until the alarm company was able to tweak their settings to match the changes being made at the same time by the water plant.

On the electrical side, a similar thing occurs every evening as the power company changes the regional source of power to our community.  Massive load changes that are seldom considered can include when a hydro system switches from providing power with their turbines all day and evening when loads are high and then switches to using those same turbine-generator systems to pump millions of gallons of water back up hill to a pumped storage reservoir overnight. That big supply becomes a big load, and the whole system must adjust accordingly. Again, it may be in another state, but it is part of your power supply.  

Literally everyone's situation will be different, within your house, building, neighborhood, community, state and so on.  Some people will have "cleaner" power at night, and others may find it is worse.  With luck, each of us will find our "sweet spot" where and when what we hear is the best music to our ears. 

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."

kijanki

They have to operate at reduced power ...

You are misreading the FCC. In the U.S., broadcast radio stations operate at authorized power output at all times although, as noted, some AM stations have lower authorized power at night and others ("daytimers") sign off at night. (There is such a thing as "pre-sunrise authorization" that some stations obtain.)

FCC regs call for the transmitter to be within 90 percent to 105 percent of authorized power output at all times. Special authorization is required otherwise and stations can be fined for failing to follow the reg.

@kijanki I believe the text you're referring to is used to calculate power (so as to avoid interference) during the licensing/permitting process. Once power (which is really ERP) is authorized, no further adjustments are allowed without application.