In the Francophone world you often come across theatres and hotels named "l'heure bleue." When clocks are depicted in suggestive paintings, they are often set at the blue hour. When is the blue hour, and why is it important to painters and photographers?

The blue hour is at four o'clock in the morning, before the opulent and busy morning has started and many people still sleep. During the blue hour, when the night plays with dawn, light has a rare quality from the sky's cold blue and the star's warm yellow light, which bathe objects with two opponent illuminants. At this mesoptic illuminance level our visual system is tetrachromatic, with rods and cones all contributing to colors appearance.

In the Silicon Valley, the light pollution is so high, that this special time of the day cannot be appreciated, but for example in the Alps, nature is still pristine and nights are dark. I invite you to experience the blue hour, and even dawn, on top of a pristine mountain. Then, please, help fighting light pollution and turn off your lights when you sleep.

By the way, five and six o'clock are not colored, they are "l'aube" (dawn), and "le levé" (get up).

In reality, l'heure bleue, this quiet time of the day when nocturnal animals have already gone to sleep and diurnal animals are still sleeping, has a different meaning to artists. That perfumes have been named L'Heure Bleue is a hint. As Félix Vallotton's 1899 "La visite" shows, it is the time for lovers to say good bye, and the time on the alarm clock on the gentleman's night stand indicates the blue hour.