64|A FONDO lower values the rest of the day. The hormone melatonin is produced only at night. As you can see, the night is not only a time to rest but a time when our body does important work that is different from the work it does during the day, but just as necessary. The temporal order performed in all the functions of the body is comparable to an orchestra, where every musician enters at the right time, under the order of the director, the SCN. The SCN generates an endogenous circadian rhythm, but to keep at the same pace that the 24 hour external cycle, it has to get in check every day. Light is the synchronizer, or agent capable of resetting, the most important biological clock in our body. If the SCN receives the light signals consistently, every day at the same time, it will be able to maintain every function in the body synchronized, between each other and with the external cycle. On the other hand, if the light signals are irregular, too much light during the night hours, for example, or irregular changes in light such as the ones caused by working night shifts, an alteration in the time structure of the body, or chrono disruption, can Light is perceived through the eyes which are the sensory organs of vision. The eyes have a covering of nerve cells: the retina, highly specialized to perceive specific wave lengths of light. The retina has different types of photoreceptors, which are cells sensible to light. Classic photoreceptors are the cones and rods. Humans have three types of cones with “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”. maximum sensitivity at wave lengths of close to 570 nm, 530 nm and 430nm, depending on the photo pigment they contain. Stimulation of the cone system enables diurnal vision or photopic vision, in color. Rods, on the other hand, have only one photo pigment, rhodopsin, with a maximum sensitivity of 500 nm. Rods are much more sensitive to light, reacting to very low intensity of light and are responsible for night vision or monochromatic vision. The information received by cones and rods is transmitted by nervous impulses to the occipital area of the cerebral cortex where the visual functions are integrated. However, light is useful for more than vision. On the last two decades, other type of retinal photoreceptors, containing the photo pigment melanopsin, have been characterized. This photo pigment is stimulated by light wave lengths of the blue band occur, and it is a risk factor to our health. Light beyond vision Of the spectrum, with a maximum of 480 nm. Even though these cells also receive information from the cones and rods, they form an independent vision