**2. Methodology and Sources of Information**

The aim is to study certain processes that occur in nature without intervening or manipulating the possible variables. For this purpose, an analysis of the physiological and psychological consequences of the absorption of light through the human visual system will be carried out through an exhaustive bibliographic and documentary review based on the proposed objectives. In this review, we undertake a study of the existing literature in the scientific community concerning the effect that artificial lighting can have on adolescents, including an exhaustive study of the literature on lighting in classrooms, the use of natural lighting, quality requirements of the interior environment in classrooms, and how lighting influences the visual health of adolescents.

The Federation of National Manufacturers Association for Luminaires and Electrotechnical Components in the European Union (CELMA) published measurements of several LED luminaires with an illuminance of 500 lx showed that none of these luminaires had blue emissions in group 2. EN 62471 classifies light sources into four risk groups: 0, 1, 2, and 3 (where 0 = no risk and 3 = high risk). The sun is classified as high risk, or group 3 [8]. Further measurements by the French Agency for Food, Environment and Health and Safety at Work (ANSES) in 2010 showed that all measured LED sources were in groups 0 and 1 according to EN 62471 UNE EN 62471:2009, the "Photobiological safety of lamps and appliances using lamps". This risk is measured according to two different methods, depending on the product being measured [8].

La EN 12464-1, the European standard for indoor lighting, has detailed the recommended values of illuminance and uniformity for classrooms (6.2 Educational Buildings) with light levels between 300 lx, and 500 lx (EN 12464-1, 2002) [9]. Figure 2 shows the blue light retinal exposure risk weighted irradiance for small sources (EB).

**Figure 2.** Comparison of the irradiance values of some lamp types at 500 lx (typical for indoor lighting) with daylight at 5000 lx (typical for outdoor lighting) [10].

Light emitting diodes (LEDs) are light sources in which light is created within solid-state materials. The light emission is obtained by the interaction of an electric field with the solid material. The physical process is called "electroluminescence". This phenomenon was discovered as early as 1907, and the first practical product based on it was created in 1962. The semiconductor material used in LEDs is selected so that it emits in the visible or ultraviolet range. Different materials produce light with different wavelengths (different colors). They have the characteristic of having a very narrow range of emissions.

The general lighting levels used in interiors via LED luminaires with cold color temperatures of 4000 K, 6500 K, and even 9000 K (whose blue components very high) are never enough to cause damage to the retina, as long as one does not look at the naked light source. The possible photobiological risk of blue light can be assessed using the criteria set out in EN 62471 [10], which have been indicated above.

From three billion years ago until 130 years ago, living organisms had one part of the day of sunlight, and other part of the day, darkness. From 130 years ago until today, thanks to artificial light, humans have experienced short periods of sleep, dim light inside the buildings where we work, short periods of time receiving natural light, bright light in the bathroom during the night, and dim light in the bedroom while we sleep [11,12].

Before the invention of electric light, humans had a "first sleep", after which they would arise and engage in tasks or go and visit neighbors. Then, they would have a "second sleep". This whole process took about 12 h. Today, we have only one sleep that lasts 7 or 8 h at most [13,14].

The study of the non-visual effects of light entering our eyes is so topical that the 2017 Nobel Prize in Physiology and Medicine was awarded jointly to Jeffrey C. Hall, Michael Rosbash, and Michael W. Young [15] for their discoveries on the molecular mechanisms of circadian rhythm control. Below are some terms that have emerged in this new branch of science.

Chronobiology is the branch of science that deals with the physiological variations of living beings called circadian rhythms, which occur approximately every 24 h, as well as the study of the processes of synchronization with the surrounding environment The variation of endogenous circadian rhythms that are determined from individual differences and psychological factors is described as a chronotype. Twenty-five percent of people are active from noon onwards, go to bed late, and then get up late; these people are commonly called "owls" [11].

Smolensky and colleagues define chronodisruption as an alteration in the internal temporal order of physiological, biological, and behavioral rhythms. If chronodisruption becomes chronic, the asynchrony, advancement, or retardation of peripheral clocks may occur [16].
