You might have read that if you took a night-picture of the Eiffel Tower, you can technically get in trouble in front of law. Actually, oddly enough, there exist certain regulations under French copyright law regarding photography of the illuminated Eiffel Tower. Everything you need to know is contained below.
While the Eiffel Tower itself is not protected by copyright-since its designer, Gustave Eiffel, passed away in 1923-copyright extends for 70 years after the creator's death, so the lights outlining the monument at night are a much different affair. The lighting design was first introduced in the 1980s and, as such, is deemed a "work of art" and remains under copyright.
That means while it is totally legitimate to take a picture of the Eiffel Tower during the day, the photograph at nighttime with the tower nicely illuminated might infringe copyright, especially when it is for commercial or professional use.
These copyright regulations are protected by the management company of the Eiffel Tower, known as the Société d'Exploitation de la Tour Eiffel (SETE). While technically you could be liable for their copyrights if you are selling or distributing a night-time photograph of the Eiffel Tower without obtaining permission from SETE, jail sentences are unlikely for individuals casually sharing photos online. Fines or court action may be brought if the image is to be used commercially without proper permission.
This is an unusual legal situation derived from the complexities and sometimes quirkiness of French copyright laws. While many people may not have such knowledge, SETE does with regard to commercial use. With every change in lighting for the Eiffel Tower, be it in a new illumination scheme or special light displays, SETE can renew its copyright claims, extending the copyright protection even further.
You are free to take as many pictures as you want of the Eiffel Tower if you do it during the daytime. While the architectural work was no longer within the copyright bar, daytime photos may be freely disseminated and distributed without one having legal qualms.
If you use or distribute the Eiffel Tower picture taken at night for work purposes, it is best to ask permission from SETE. Often paid, this way you remain on the right side of the guidelines.
Reference Links: Wikimedia Commons Feature Discussion on the Eiffel Tower's Night Photo
In summary, while clicking that picture of the Eiffel Tower at night will not put you behind bars, using or reproducing it without permission may land you into trouble someday in any country with a real copyright law. So, do know the rules if you plan to take some pictures of how beautiful Paris becomes at night!