Although there is very little naturally occurring infrared light available at night, there is often a surprising amount of infrared light from man-made sources bouncing around.
As a result, it turns out that, to my surprise, taking IR photos at night is absolutely possible, often with very interesting results.
Such as these, all of which were taken using Hoya R72 filters screwed onto various C/Y lenses in combination with a DIY-converted full-spectrum Samsung NX500 mounted on one of my FrankenKameras:
Of course, I also enjoy doing IR photography during the daytime as well, especially of the handheld variety as I walk around with my dog at midday, with its harsh, Arizona summer sunshine. All of these photos were taken using a Hoya R72 filter screwed onto a Contax N 24-85 zoom in combination with the same DIY-converted NX500 I used for the nighttime photos, then subsequently converted to B&W during post-processing:
P.S.: To my surprise, almost without exception, all of my vintage C/Y lenses, as well as my Contax N lenses, work very, very well for IR photography in combination with my modified NX500 and a Hoya R72 filter. I can't speak to whether they will work equally as well when used with other camera bodies or different IR filters or focused at different distances or using apertures other than f5.6 and f8 (my usual choices), but they're priced right, not to mention compact and lightweight ... a win-win all around!
And as a final bonus, here's a five-panel pano I took handheld using the above CN 24-85/NX500 combo and then stitched together using Photomerge in Photoshop CS6: