During the daytime, the D-layer of the ionosphere is extremely active. The D-layer is an absortive layer, although the absorption decreases with increasing frequency. But on 80 meters, the absorption is high.
At nighttime, the D-layer disappears, so the refractive layers (primarily the F-layer) are exposed. This allows propagation on 80m at night most of the time, assuming the MUF of the F-layer is high enough (which it usually is for 80).
Most amateurs think of 80m as a "local" band, because it is extremely difficult to have a high-efficiency, low radiation angle antenna on that band. But worldwide propagation is available for those who are able to construct a suitable skywire. I have a 1/4-wave sloper on 80 with the top end a about 40 ft. This is not a good DX antenna on 80.
Returning to daylight hours, note that there is some "skywave" possible during the day. But the attenuation of the D-layer is such that your signal can only go through the D-layer a few times before it is reduced to an unusable level. That is why communication distances are limited to a just a few hundred miles during daytime.