Day for night, also known as nuit américaine ("American night"), is the name for cinematographic techniques used to simulate a night scene; such as using tungsten-balanced rather than daylight-balanced film stock or with special blue filters and also under-exposing the shot (usually in post-production) to create the illusion of darkness or moonlight.
Historically, infrared movie film was used to achieve an equivalent look with black-and-white film.
A great way to achieve this effect is to tune the white balance of the camera to a yellow source if there is no tungsten setting. Another way to make a more believable night scene is to underexpose the footage to the desired degree of night/darkness. This depends on the amount of light shown or believed to be in the given scene.
The title of François Truffaut's film Day for Night (1973) is a reference to this technique.