*3.1. Studies on the Non-Visual E*ff*ects of Light Absorption through Our Visual System*

Stevens and Zhu [17] observed that the sun is our primary source of light during the day and that for millions of years sunlight has shaped mammals' endogenous circadian rhythms including when to wake up, body temperature, metabolism, oscillations of gene expression, and hormone production throughout our bodies. Electric light, in contrast, is dim and alters all aspects of our internal circadian rhythms. Its intensity and spectral content are often not adequate during the day for proper circadian resetting and are too great at night for "true darkness" to be detected.

In 1975, the scientist Richard J. Wurtman wrote an extensive article talking about the effects of light on our internal organs, such as the ovaries, and tissues, such as the breasts. This publication was visionary because of the knowledge of the scientific community at that time [18].

Guido et al. [19] established that the retina contains a biological pacemaker that influences the entire circadian system. Glickman et al. [20] stated that an illuminance of 2500 lx is necessary to suppress nocturnal melatonin in humans, but it was later determined that under certain conditions, such as below 1 lx, melatonin can be suppressed in humans.

There is a growing trend in the scientific field to a consensus that exposure to light influences many psychological processes through at least two pathways unrelated to the phenomenon of vision [21]. The best-known pathway is related to the regulation of melatonin secretions by the pineal gland [22]. This pathway controls circadian rhythms. Exposure to light at night, particularly at short wavelengths, suppresses melatonin and influences insomnia. The other pathway acts on the level of alertness by activating a mechanism separate from that of melatonin suppression, during which cortisol is secreted [21].

Given the large amount of data that is gradually appearing on the positive and negative effects of artificial light related to health, Erren et al. defined photohygiene as exposure to light under the optimal conditions of periodicity, quality, and quantity [23].

A study carried out by Cho et al. [24] stated that sleeping with the lights on causes acute negative effects on the structure and quality of sleep. A later study showed that these negative effects could affect aspects related to memory by producing corneal levels of less than 10 lx. The experiment was conducted with a 5779 K LED source with a diffuser [25].

TheWorld Health Organization's (WHO's) International Agency for Research on Cancer (IARC) has classified "shift work involving circadian disruption" as probably carcinogenic to humans (group 2a). Continuous light contributes to acute confusional syndrome in adult intensive care units, where environmental constancy is the norm [26].

Dim light melatonin onset (DLMO) refers to the onset of melatonin secretion under low light conditions. The human body is programmed for this moment to occur when the sun is setting. Graham and Wong [27,28] noted in their study that intense blue light cancels the night peak 10–20 min after exposure, which returns to its initial value after 40 min once the stimulus is removed.

In relation to the time of day, exposure to light can generate clock advances or delays as defined by the phase response curve (PRC), so bright light at the beginning of the biological night (from the start of melatonin elevation to the time when the minimum body temperature is produced) generates a phase delay, while in the morning (from the minimum body temperature to 8 h later), it produces an advance. Epidemiological, clinical, and experimental studies with animal models show that the chronodisruption produced by artificial light during the night may be related to pathologies such as the increased incidence of metabolic syndrome [29], cardiovascular disease [29], and cognitive and affective disorders [30].

Exposure to light at night (LAN), reduced or inadequate light intensity during the day, or decreased contrast in the light–dark cycle all contribute to chronodisruption (CD). The cognitive decline, affectivity, behavioral and sleep disturbances, and limitations in the daily activities of elderly patients with senile dementia and their caregivers have been associated with alterations in circadian cycles [31]. Artificial light at night (ALAN) is drawing the attention of researchers and environmentalists for its ever-increasing evidence on its capacity for "desynchronization" of organismal physiology [32]. Obesity is a common disorder with many complications. Although chronodisruption plays a role in obesity, few epidemiological studies have investigated the association between artificial light at night (ALAN) and obesity. Since sleep health is related to both obesity and ALAN, Koo et al. [33] investigated the association between outdoor ALAN and obesity after adjusting for sleep health and the association between outdoor ALAN and sleep health.

Simón and Sánchez [34] noted that about 20% of people in today's society spend most of their time during the day indoors under dim lighting, with low physical activity and irregular, short sleep cycles. The authors suggest that these factors could contribute to the prevalence of chronodisruption possibly facilitating pathologies such as cancer, intestinal conditions, metabolic syndrome, cardiovascular diseases, mood disorder, and cognitive impairment. Chronodisruption may also adversely affect melatonin and cortisol levels. Cortisol is a regulator of stress-related functions [34]. In 1998, Sterling and Eyer [35] defined the role of allostasis as "maintaining stability through change" and noted that cortisol secretion and stress are a "body's adaptation to an unknown situation that must be transient and therefore blocked or stopped". The authors comment that night shift workers have a high risk of circadian disruption and, therefore, hormonal alterations. Similarly, Mirick et al. [36] suggest that a low level of urinary sulfatoximelatonin is related to working at night, which results in higher levels of cortisol.

Stevens and Zhu [17] state that light is a regulator of psychology and behavior and that its effects have evolved over millennia throughout which illumination has provided reliable information about the time of day. The authors claim that the advent of electric light has now altered this relationship with patterns of light exposure reflecting personal tastes and social pressures. It is important, therefore, that non-visual light effects be incorporated into lighting design. For example, one might ask to what extent existing architectural lighting replicates the biological effects of natural light, how lighting could be used to minimize the harmful effects of shift work while promoting alertness and safety, or how light therapy could be optimized. The lighting industry and scientists have begun research in this direction. They argue that we must first determine how light impacts human behavior and psychology. There are two different techniques for measuring light and there are different scientific criteria to determine which of the two is the most suitable: radiometry (quantitative analysis) and photometry (qualitative analysis).

Several studies have observed that humans are adopting increasingly nocturnal lifestyles, both for work and for leisure, which has resulted in the night becoming excessively illuminated, while we spend most of the day in poorly lit interiors. This results in an increasing gap between our habits and the natural synchronizers of the circadian system. Chronodisruption or circadian disruption (CD) is the physiological price of exposure to light at night [16,17].

It is well established that light affects both visual and non-visual systems. Little attention has been given to testing the effects of light on building occupants' non-visual responses, and, consequently, lighting specifiers have been offered little guidance on the design and application of lighting for non-visual effects. A study conducted by the Lighting Research Center, by Figueiro et al. (2019) [37], helped to fill that gap through field-testing of light exposures from a novel luminaire designed to promote entrainment and alertness throughout the day in actual office environments. The data supported the inference that light exposures, when properly applied, can promote circadian entrainment and increase alertness.

Recent research has shown that exposure to bright white or high blue light stimulates alertness, but these effects are not seen in tasks that demand a high cognitive level. Individual and psychological differences have been taken into account to explain the variability in the cognitive effects of light. Sensitivity to light depends on individual differences in the PER3 gene clock involved in sleep–wake regulation, age, the cognitive domain, and task difficulty [38]. Some authors claim that exposure to bright daylight indoors can result in positive vitality, alertness, and help promote a healthy, active day. These studies reveal that bright light induces improvements in alertness when healthy participants are experimentally deprived of sleep or light prior to exposure to this indoor light [39].

Experimental studies have shown that the magnitude and duration of non-visual light effects depend on previous light doses [40]. Exposure to bright indoor light would induce weaker non-visual effects in spring than in autumn and winter [39].

Angel Correa et al. [38] observed that bright white light or light enriched in the range of blue increases alertness but that this is not effective for high cognitive level tasks, such as sustained attention to task response (SART). The authors observed that the results varied greatly depending on the individuals and their previous states of alertness, with higher results among those who previously had a better state of alertness or vigilance.

Wright Jr. et al. [41] conducted an experiment in which they recruited eight participants (two women and six men) aged around 30 years old whose circadian cycles were previously studied at their work and home. The participants then spent a week camping in the mountains without electricity. Among the eight subjects, there was a wide range of chronotypes (i.e., larks and owls) and sleep times. The beginning of melatonin release is approximately 2 h after sunset. Due to the habits of modern life, this release now occurs later. After a week of camping, the beginning of melatonin secretion occurred closer to sunset and elimination occurred at dawn, aligning larks and owls more closely to the duration of natural light. In their conclusions they state that "increased exposure to sunlight may help reduce the health consequences of circadian disruption".
