Recently, my screen brightness has switched down to 29% in the evening

For a few days now (can’t say exactly when this first happened), the brightness of my monitor has been switching down to 29 ½ every evening, although I haven’t activated anything like this anywhere in the Plasma settings.

You say its not, but have you actually checked the “Night Light” settings?

System Settings > Appearance & Style > Colors & Themes > Night Light

If its reliably doing it at certain times … it sounds a lot like it would be that.

I had never activated this before, and none of it is activated now.

Anything similar like redshift ?

Some machines have ‘adaptive brightness’ or similar settings as well.

(but if this is a new thing its maybe less likely to be something like a BIOS option)

My monitor is an old Dell from 2018, it didn’t have Redshift and I didn’t set anything like that in the settings. In addition, the color temperature does not change, only the brightness (from the default 75% to 29%). Nothing like this is activated: