I rigged up a simulated sunrise about 20 years back with a regular light, a computer, and an X10 controller back when I slept in a room without windows and wanted a more-gentle way of waking up than an alarm clock. So I totally get wanting to simulate a sunrise.
But why do you want to simulate a sunset?
Also, you might want to ask on !homeautomation, as they specialize in this sort of thing.