Nocturnal lights and objects make up 55% of UFO reports. 87% of these nocturnal lights, those of strangeness 1, 2, or 3, match Vallee's FB1 flyby one category: a simple sighting of a UFO traveling in a straight line across the sky. These may often be planets, aircraft, re-entering space junk, meteorites, migrating birds, or other conventional phenomena that for whatever reason don't look right to the observer.
10% of nocturnal lights, those with strangeness 4, match Vallee's MA1 category: they make maneuvers. These can include hovering, darting off towards the horizon or vertically at great speed, and making 90 degree-maneuvers with seemingly impossible rapidity. They can also include lazy terrain-following, seemingly barely-in-control wobbling and 'falling leaf' motion. Sometimes a larger object separates or spins off other objects. Sometimes formation, v-formations, or entire fleets of objects pass overhead. In any case, they surely do not match the usual behavior of aircraft, or of meteors or re-entering space debris.
3% of nocturnal lights, those with strangeness 5, match Vallee's FB2 category: there is physical evidence of the sighting. This almost always means a photograph was made. These are generally not impressive in terms of the photograph. But as evidence, demonstrating the maneuvers, they are often extremely impressive.
In the last few years several observatories and amateurs have set up full-time low-light cameras, linked to computers with motion capturing software, such as UFOCapture. These continue to capture nocturnal migrating birds, satellites, meteors, upper atmospheric sprites, lightning - and some objects that can't be explained.
The Hassdalen project has had an instrumented station in place at a location well known for 'ghost lights' for over a decade. Despite an array of cameras, spectroscopes, magnetometers, and other instruments, and dozens of sightings, no definite conclusion has been reached.
So it seems that whatever they are, like other UFO phenomena, they can be experienced, photographed, and behave in consistent patterns - but cannot be proven.
Nocturnal Lights Chronology
Nocturnal Lights by Location