Bill Denton wrote:

There are only a limited number of AM frequencies available in the US, and
these frequencies are shared across the US.

And Canada and Mexico too.


For example, a station in
Monroe, LA could be on the same frequency with a station in Las Vegas, NV.
This will work because the power of the stations is limited and they are
geographically separated.

But at night, an AM signal goes farther than it does in the daytime. It has
to do with skywaves vs. groundwaves, but I have long since forgotten that
stuff. So trust me, the signal goes farther at night than it does during the
daytime.

So, the stations must reduce power at night to avoid interference with other
stations on the same frequency.

Yes and one of those stations will have the "priority" at night.
Commonly referred to as clear channels. For example WCCO 830 in the
Twin Cities is a clear channel. They broadcast at 50,000 watts 24/7.
All other stations that broadcast on 830 must turn their power way down
at night because WCCO essentially owns the freq at night.