**1. Introduction**

### **1.1 Circadian rhythms**

Circadian rhythms have several characteristics that are important to analyze. Under constant environmental conditions, these rhythms persist in a period close to 24 h. However, some of the physiological functions have different periods, so that circadian rhythm are considered as part of a multioscillative system [1]. However, the circadian rhythms of the different functions remain synchronized by a central pacemaker, located in the suprachiasmatic nucleus of the hypothalamus [2]. A set of genes involved in the generation and modulation of these rhythms was verified [3]. It was found that some environmental events keep circadian rhythms synchronized with environmental cycles. These synchronization agents are lighting cycles, room temperature, food availability, exercise, and social stimulation. Ambient lighting cycles are the most effective synchronize circadian rhythm, both in animals and in humans [4].

Three neurophysiological pathways participate in the responses of humans to light. The first clue includes two types of receptors in the retina, rods and cones, which respond to the intensity and frequency of light. These receptors are connected with bipolar and amharic cells, and ganglion cells that connect to neurons in the lateral geniculate nucleus of the thalamus and neurons in the occipital cortex. This nervous

path participates in the analysis of shapes, colors, and images. The second pathway involves ganglion cells in visual connections with nuclei of the superior colliculus of the thalamus midbrain, pulvinar nuclei, and the inferior temporal cortex. This nervous path participates in the identification of space. The third visual pathway includes a group of retinal ganglion cells that respond to light, have connections to the suprachiasmatic nucleus and participate in the mechanism of synchronization of circadian rhythms. These retinal neurons are known as intrinsically photosensitive retinal ganglion cells (ipRGCs), retinal ganglion cells give photosensitive (CGRP), or retinal ganglion cells that contain melanopsin (mRGCs) [5, 6].
