"the high/low freqs match the height of the sun in the sky: highest freqs at noon, lowest freqs at midnight"
HF works by bouncing signals (refracting anyhow) off the ozone layer, activity within which is driven by the sun. It lags the actual position of the sun though, MUF's are at their max mid afternoon, at a min before dawn. It is the activity between you and the station you need to consider, not just overhead the ground station.
I used to do International HF many moons ago in Perth, West Australia. Our transmitters were in Perth, but our recievers were miles away near Pearce. If you transmitted while disabling the reciever mute you got a nice squeal to tell you the radio worked. Near the end of a long winter night the higher freq recievers couldn't 'hear' their transmitters! On the other hand an aircraft checking the radio on 3 MHz on the ground at Perth in the afternoon could sometimes not be heard.