66|A FONDO system that has been denominated “Non-Image-Forming System” (NIF). The NIF activates a series of brain structures that produce other physiological responses, such as the circadian synchronization prompted by the action of light on the SCN, or the inhibition of the secretion of melatonin. Furthermore, it helps modulate sleep, alertness, and even humor and well- being. This system is also responsible of the photophobia that sometimes happens with migraines, or the sensitivity to bright light that some people have. Melatonin, the nocturnal hormone Light Information reaches the SNC through the retina, adjusting the circadian cycles to the external cycles. The SNC in turn sends information to another brain structure, the pineal gland that secretes melatonin hormone. This substance is completely conditioned by environmental light. Melatonin is secreted in a circadian rhythm that presents low levels during the day and high levels at night, but only in the presence of darkness. If during the night a person is exposed to light the levels of melatonin diminish drastically. The secretion of Melatonin is, to living beings, the measure of night time. It has special importance for the adaptation to the seasons of the year; winter days, with their longer nights, at least in a natural setting, will stimulate the production of more melatonin, which in turn signals to the body the time of year. Melatonin not only acts on the SNC helping to stabilize the internal clock, it also affects the immune system. It is antioxidant and neutralizes free radicals. This is why “light during night”, by stopping the production of this substance, can contribute to start or exacerbate disease. Melatonin levels diminish in direct proportion to the intensity of light, and also depending on wave lengths. Blue band light inhibits the secretion of nocturnal melatonin more than orange band light. Artificial light can modify the stability of the circadian rhythms Maintaining the stability of circadian rhythms is important for health. To achieve it, the basic principal is to keep a good light contrast between day and night. Not enough light during the day or too much light during the night can alter the internal temporal order, constituting a serious risk to health. Circadian maladjustment has been associated to sleep disorders with the consequences it brings, and also to a higher risk of developing cancer, mood alterations and metabolic diseases such as obesity and diabetes. Some of these disorders have been attributed to low levels of melatonin. Today, there is enough scientific evidence to know that the amount and the type of light, as well as the times it is received, can alter or help sustain the circadian rhythm. The circadian system is especially sensitive to the light it receives at the beginning and at the end of the day; the light it receives at the end of the day interferes the most with sleep and the secretion of melatonin. As it was mentioned before, the blue band affects the NIF the most. In general, bright light, including de blue component spectrum, tends to improve alertness during the day; it stimulates cognitive faculties and it even improves sleep during the night. On the other hand, if used during the night, bright light disrupts the circadian system, it hinders and delays sleep, inhibits the secretion of melatonin and increases body temperature and the nocturnal levels of the hormone cortisol. Therefore, to preserve the circadian rhythms, light in the evenings should be soft and of low color temperature (yellow, red); light from the blue band should be avoided. The effects of light could also be caused by the screen color of electronic devices. There have been studies that show the effectiveness of screen filters for the blue wave length