there is an easy solution to your problem, that at least works for me pretty good when working on a day-2-night-shots within the same rendering:
1. overdo your lights, a lot ... so if your light bulps usually would have intensity 1, make them 10 or even 15 (yes they are very bright now), same for every other light / sky / lightgroup / group of same light ..
2. set up your sun as you would for a normal daylight shot
3. do your normal rendering and let it denoise
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4a. daylight:
reduce your sky value and disable/reduce interior/exterior lights that would not be "on" or very visible during the day in lightmix - change sun/skycolor if needed - daylight done
4b. sunset/night:
reduce/lower your sun intensity to whatever suits your needs, same for the sky and all other lights, change intensity/color/temp in lightmix.. - sunset done
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the whole reason why you had to overdo light intensity on all the lights, even tho you wont use them that bright, is that after the denoising, the image hase a lot more information on how/what every single light/lightgroup would illuminate. that way you reduce fireflys, brunspots, brightspots and your lights have a very soft falloff.
its already been said, using a value of 1000 on a light may be too much, but reducing it on lightmix to 250 for a nice look, is perfectly fine..
give this a try on low res if you dont belive me right away ;)
