It must have happened to you: you are in your bed at night, in the dark, eyes wide open. You have tried everything: counting sheep, meditation, breathing, melatonin. Nothing works. There you are, wide awake, ready to run a marathon in the middle of the night. You're having insomnia, or you could say: une nuit blanche. Passer une nuit blanche - literally to spend a white night - means staying awake all night, whether you choose to (you're studying, working, partying...,) or because you can't sleep, even though you'd love to. Now, why exactly would that sleepless night be white? Why not purple, or blue?
As usual with old idioms, there is no certainty as to its exact origin. There are, however, 3 hypotheses that are rated most likely, with #2 and 3 being the most likely:
1- In French, the adjective blanc / blanche - white - often refers to "lacking in something, being incomplete or deficient." It is somewhat similar to the English "blank" - as in blank vote, blank document, blank wall - although the English word more often describes something, or a space, that is bare, empty or plain.
Examples in French:
une voix blanche - a toneless voice, a blank voice
un examen blanc - a mock exam
un mariage blanc - a paper marriage, a marriage of convenience
Some linguists suggest that, following this logic, a sleepless night would rightfully belong to the category of "incomplete or deficient" experiences. And white, being colorless, describes that category best.
2- The most widespread explanation, however, is a bit more fun. It has to do with knights in the Middle-Ages. It turns out that those knights, right before their dubbing, would have to stay awake all night and pray ....in an all-white outfit! Hence the idea of a white knight spending a white night.
3- Finally, there is the Russian origin, which is even more fun. This one takes us back to the 18th and 19th centuries, when the tsar resided in St. Petersburg. The Russian royal court was very popular with the French nobility, particularly during the summer months. In summer at that latitude, the sun never sets completely, and neither do the inhabitants. At the court, parties and balls were in full swing all night long, night after night. Those were called the "white nights" in Russian - see also the novel The White Nights by Fyodor Dosteievski. The expression passer une nuit blanche would have been brought to France by those merry night owls returning from Russia where the summer nights were never dark.
Elle est du style à passer une nuit blanche pour finir son travail.
She is the kind that would stay up all night to finish her work.
Passer une nuit blanche, ça nous arrive à tous un jour ou l'autre.
We all experience insomnia from time to time.