Well, of course you have to go friendly with Synedrion or steal the tech from them. If not, then you can even end the game and still don’t have access to this night vision module. Time is relative 
In my experience nighttime actually begins about 3 or 4 hours after the sunlight fades from the geoscape.
… so a fair bit past that line.
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Oh really? I have to admit that I never looked that exactly, I rather estimated it.