thanks to the direction you pointed me to i found this - thanks.
yawn at 3AM click FLASH “I’m blind! I’m blind!” dimm to 30% “And now it’s even more dark!” 
Relay is this aeotec nano dimmer - but i think i found a solution for my problem.
There is a setting “maximum brightness level” which i could use.
I have to figure out how it behaves if it switches back to 90%, but the REST-API should be something to start with.