It's important to remember that airplane windows are different from ordinary house windows. For one thing, the panes are not flat but instead slightly curved, which can alter and distort images seen in them.
Furthermore, objects outside the cabin are viewed not through one pane but instead through at least two transparent panels of glass and plastic; this creates an overlaying double reflection of lights from inside the cabin.
The image is reflected once by the plastic near the seat, and once more by the thick laminated glass on the inside of the window itself, a few inches beyond. The result is a reflection that often appears as a duplicate image, depending on the angle of the viewer.
Indeed, this effect can be seen in the 'UFO' video. At several places in the video (including 0:18 and 0:41), this duplicate ghosting effect is apparent; though the image is moving and blurry because of the unsteady camera, the same pattern of lights can be seen overlapping each other.
Mr. Ruiz moves his hand toward and around the window, apparently trying to rule out the possibility that the image is a reflection from within the cabin. However his hand only blocks reflections coming from the extreme left of the window; it does not rule out reflections coming from above and behind him - which is where cabin lights would be.
At one point the lights suddenly disappear, likely because a passenger or airline attendant passed (unnoticed) behind the cameraman and in front of the cabin light being reflected.
Further support for this explanation can be seen when the video shows an outside wing light, which looks very different than the "UFO." This is what a true light outside the fuselage looks like: one single coherent light (complete with minor lens flare), not a double-reflection of small, scattered duplicate lights showing no lens flare.
Because interior airplane cabin lights are designed to point downward and not across the aisle, it makes sense that the reflected light would not be one single light source (such a spotlight effect), but instead two or three smaller, irregular lights incidentally reflected - and duplicated - in the window.
Then, of course, there's the most obvious reason that the light in the video is inside the airplane and not outside in the night sky: the pilots would have seen it.
Any flying saucer or unknown aircraft flying close enough to the plane - not to mention of the apparent size of the object (given its brightness) - would have posed an imminent threat of collision. Not only would the pilots have seen the 'UFO,' but likely taken evasive action to avoid it. Yet there is no report of the pilots - nor any other passengers, for that matter - seeing the supposed UFO.
It's likely that the UFO is merely an optical illusion created by one of the passenger's lights being reflected from the other side of the aisle at the right angle, and being noticed by the cameraman.
Ruiz's state of mind could have contributed to misperceiving the image: He said he was exhausted "after three long days in New York City and after a long wait at the La Guardia airport, followed by long hours inside the airplane." It's not hard to see how a cabin light could have been mistaken for an exterior UFO if you are tired and bored on a long, late-night flight.
There are, of course, other possible explanations, including a computer-generated hoax, ordinary lights outside the aircraft, or even an extraterrestrial spacecraft. In this case, it seems most likely that the mystery light is an Unidentified Reflected Object.
Ruiz posted a note on Dec. 27 stating that "This footage is still under reviews. Please be patient I will keep you posted with the latest. Thank you very much." In an earlier posting Ruiz stated that "The actual footage in its original content was sent to the FAA for further review. UFOlogist and investigators please be patient. Let FAA do their review in fairness to all."
It's not likely that the Federal Aviation Administration is devoting their top experts to analyzing his home video. After all, it's not the FAA's responsibility to make sure that all airline passengers are able to identify any lights seen out a window during a commercial flight.