From https://en.wikipedia.org/wiki/Skywave : > Because the lower-altitude layers (the E-layer in particular) of the ionosphere largely disappear at night, the refractive layer of the ionosphere is much higher above the surface of the Earth at night. This leads to an increase in the "skip" or "hop" distance of the skywave at night.

Also came across this interesting page: https://www.fcc.gov/media/radio/am-stations-at-night