Photography time-lapse video

I have a nikon D90 (dslr) and i'm shooting a time-lapse of the sunset. I don't want to use the automatic mode because it will end with the video having the same brightness/light. I'm trying to shoot the timelapse with manual mode so i can get that "effect" of the time-lapse video getting darker gradually. My question is what is the best settings i could use to get this "effect" in terms of shutter speed, aperture and iso.

What you're trying to do is called "The Holy Grail" of time-lapse for a reason. That said, there are a few options for day-to-night videos.

Post processing - record a day video and a night video then do a cross-fade edit. This looks good and is probably the most common method.

Bramping - stands for "bulb ramping" and is available through some add-on devices and software. You can adjust the exposure throughout the shoot and then use LRTimelapse or Panolapse to smooth out the transitions.

Automatic - There are really only two options if you want to avoid flicker--aperture priority and automatic ISO. The ISO method is preferable if your camera can go high (~6400) without too much graininess. Either way, you'll probably have to use adjustment software to smooth the transitions.

If you're doing a gradual transition, make sure to shoot in RAW or the colors will be off. Both LRTimelapse and Panolapse will let you smooth color transitions if you use RAW files. You can let me know if you have any other questions by using the contact form link below.