I will usually acknowledge some road manners by at least raising a roady style twitch of the finger
Dunno about you but I can’t see through other drivers’ windscreens at night 😉
Hence a quick flash instead seems polite. Most likely, the people who DO acknowledge you by day are the ones that flash at night instead.
Calm yourself by telling yourself that those who seemed not to say thanks at night really did, but they just waved without thinking properly and you didn’t see it. Because, y’know, it’s night, and dark. Better for your blood pressure than telling yourself that those who do say thanks are the ones that normally don’t… this makes no sense. They’re not out to annoy you specifically.
I’m with your Mrs 😉