Found this book after searching the internet:
“Die wissenschaftlichen Grundlagen des Rundfunkempfangs”, 1927.
There is a lecture by Prof.Dr. R. Rüdenberg to find.
On page 226, a diagram shows the field strengths over several days.
I translated with the help of “google translate”
4. Influence of the atmosphere.
When transmitting over long distances, for example … from America to Europe, one observes great differences
in the intensity of day and night reception.
In general, daytime reception is poorer and there are particularly strong fluctuations in sunrise and sunset at
the transmitting or receiving station itself.
Fig. 45  shows a series of measurements of the course of the field strengths from distant stations, recorded over several consecutive days.
A fourth reason might be:
The background noise decreases at night, you can do that see the color change in the waterfall diagram of a Web SDR.
(while observing “cutler”,USA during the night)
: Rem: “W.Q.K” means “Radio Central, Long Island (18,788 KHz), see “wikipedia”