Surely the sun has some measurable effect on the air temp, even at cruising altitudes.
I do not think so; I have never noticed, read any or seen any data linking change of temperature at higher altitudes in relation to the sun setting or sun rising. Nor is there any real difference between summer and winter.
The sun is the main energy source for most of the planet’s weather. It provides the initial heating (direct light) and cooling (darkness) for the planet’s continental/maritime/tropic/polar air masses that then mix. So, yes the sun has a huge effect on temperatures, including temperatures at altitude. However a given sunset at FL370 does not result in an immediate decrease in temperature as the sun dips below the horizon.