How can I switch to/from dark mode depending on "darkness" instead of sunset? On BigSur the Sunset-Detector works but it doesn't make sense, because in the real world darkness arrives a bit earlier than sunset. It hurts my eyes, because I stay focused on my work and don't notice that it's dark already.
How can I have macOS toggle dark mode earlier/later than sunset/sunrise for whatever location I'm currently in?
Basically you (and third party developers) don’t get to automate this since it’s a private API. This article explains the situation in great detail and offers three tools to get you close to customizing it.

https://setapp.com/how-to/use-night-shift-for-mac


First, turn the night shift off under “Schedule” in the night shift menu. This disables it schedule from activating, but doesn’t disable the feature.

I wonder if your setup is the ambient light sensor changing and not the schedule, though.

It will activate when it notices your environment is darker than normal, and disable itself when you’re back to daylight or bright environments.


Keep in mind this option will trigger automatically, which can be frustrating if you’re in a spot where the light fluctuates.

Short of using an automation tool to define your own preferred schedule or just automate toggling this manually, you can use Apple’s implementation or just take breaks and manage your lighting external to the display.
Some excellent automation tools include

https://www.obdev.at/products/launchbar/index.html
https://www.keyboardmaestro.com/main/