This is one case where it's useful to steal ideas from Hollywood, who have been doing this for decades.
Typical hollywood night scene, also related question from Movies.SE
The picture above was filmed during the night time, but it's not actually a dark picture. Notice how the actors' faces are very well lit, although the sky is pitch black and the road is very bright (which you wouldn't normally expect). Studying similar shots will give you some clues to how you can fake a night scene while still allowing your viewers/players to actually see.
Tint things blue/purple
This one's easy to guess; since sunlight is usually yellow, the spectral opposite of that happens to be bluish/purplish. That is, take neutral white light, remove yellow, and you end up with a blue/purple colour.
Since our night vision uses rods which can't see colour, reducing saturation emulates this effect.
Increasing contrast has the effect of making bright things brighter and dark things darker. This simulates reality where, in dark conditions, bright things like artificial lights look much brighter in comparison, whereas we lose the ability to see dark details.
And also darken things that are typically bright during the day but dark during the night. This actually goes a long way towards the illusion and seems less "artificial" than tricks like making your scene blue. In the old days, with black/white cameras, they took advantage of this by filming during the day with a red filter over the lens, which conveniently darkens the blue sky but little else.
Hey look it's night time. - Bret Victor - Inventing on Principle
The corollary of that is to brighten things that are dark during the day but bright at night. Think artificial light sources like flashlights, campfires, computer screens. Exaggerate their brightness.
Add light sources from "unnatural" angles
If you don't have the luxury of actual light sources (like full moon, or streetlights) in your night scene, you still need to add some light so viewers/players can see, but make them cast light in funny angles so they aren't associated with the sun and break the illusion. Daylight typically comes from above at a slight angle, and this trope extends everywhere, even in UI where "3D" buttons are typically brighter at the top and left edges, and drop shadows extend down and right. For night time, reverse those angles. Have lights illuminating from below, or behind your subjects. Behind is particularly neat as it shows silhouettes but leaves features undefined, mimicking the ominous dark.
Some night scene; note the light angles and the well-outlined hat and afro
A lot of these tricks come from a filmmaking technique known as "Day for night", which is where a low-budget film will shoot night scenes during the day but use a number of filming tricks and editing to make it look like night. The relevant tricks I've already mentioned above but you can read on here: http://www.videomaker.com/article/10368-shooting-day-for-night