In recent times, the research in the field of non-visual effects of light has produced significant results, and the importance of the corneal illuminance for lighting design purposes has gradually increased. In some cases, it has been identified as the vertical illuminance at eye level, and its importance has emerged with reference to various factors, such as melatonin production. The latter has beneficial properties on the immune system, but above all on the regularization of the circadian rhythm, namely the cyclical variation of our natural and biological activities. It is precisely for this reason that the EN 12464-1 standard suggests the maintenance of an average cylindrical illuminance (average value of vertical illuminance levels on a cylinder) of not less than 50 lux for indoor environments and, in particular, more than 150 lux for places such as offices, meeting rooms, and classrooms. Thus, the lighting aspects play a central role in the maintenance of healthy circadian rhythms: the right amount of light during the day and a decrease in the intensity of the evening light stimulus can lead to a sort of synchronization of the human circadian system with the time of the place where one lives. They can also promote the balance of the so-called “endogenous clock”, which varies, depending on the individual, between 23.5 and 24.7 hours. The situation changes radically if exposure to light during daylight hours proves to be insufficient and if the night-light should be inappropriate. In this case there would be an interruption of the circadian rhythms, which, if perpetuated, could lead to problems such as sleep and metabolic disorders, diabetes, cardiovascular diseases... In particular, the question of corneal illuminance is still unclear and is subject to research. In this regard it was emphasized that, in industrialized countries and during daylight hours, the amount of light to which the occupants are subjected may be too low.