I thought Solar UV deionized the skip layer during the day, which is why AM band signals travel farther at night?
No, solar UV ionizes the skip layer during the day down to lower altitudes, leading to refraction of AM band signals from those lower altitudes back to the ground closer to the transmitter than they would at night. At night, the ionized layer is higher, the refraction takes place at higher altitudes, so the signal hits the ground farther away.
There is another effect, too: The higher ionization during the day also leads to increased absorption (attenuation) of the AM band signals at even lower levels of the ionosphere (the D layer) than those at which they are refracted. The D layer disappears at sunset, so absorption by this cause goes away, increasing the received signal strength at distant locations.
The above behavior is for the AM broadcast band (~1 MHz). Above around 10-13 MHz, the situation reverses; during the day, these higher frequencies refract from layers at higher altitudes and suffer less from absorption (the absorption goes as an inverse square of the frequency), so they travel great distances, while at night, there is insufficient ionization to refract the signals back to ground, so they continue out into space and are lost. And above around 20-50 MHz, depending on the state of the sunspot cycle, there is insufficient ionization even during the day to refract the signals back to ground, so one has to resort to secondary mechanisms (e.g., ionization trails of meteors) for long-distance propagation.
Typically, typically. The above is a gross generalization: The effects of the ionosphere on radio waves depends on their frequency, their polarization, their direction and location relative to the geomagnetic equator, the time of day, the month of the year, the status of the sunspot cycle (solar wind), the magnitude of the Earth's magnetic field, the magnitude and direction of the magnetic field in interplanetary space, and eleventeen other factors. Radio propagation prediction software (e.g., VOACAP) deals in probabilities, not certainties.