62|A FONDO In the field of neuroscience, investigations have been conducted on neural pathways that produce physiological reactions to light beyond vision. Having light available 24/7 has generated new behavior patterns, different work schedules and more activity shifted towards the night hours. But are people ready to receive light at any given time of day? Circadian rhythms To understand the biological basis of the influence that light has in humans, the environmental conditions of the planet in which they have evolved as a species, has to be taken into account. Earth spins on its axis every 24 hours and around the Sun in 365 days, approximately. These cyclic movements produce drastic changes in the environment which have conditioned life on earth. Among these conditioners, light stands out for its predictability. During the day, solar light is intense and during the night light is soft, the brightest thing would be a full moon or a starry sky. Through the year, variations in light repeat themselves with dependability; longer days in summer and shorter in winter. If evolution of livings beings on the planet owe their adaptation to environmental conditions, then adaptation to changes between darkness and light has to be a key factor for the survival of the species. In the course of evolution, organisms developed an internal structure, a biological clock, genetically determined, that varies within a period similar to Earth’s rotation, and which is capable of transmitting this information to all its functions. Thus, the physiology and behavior of living beings manifest in predictable variations every 24 hours, circadian cycles, that can be detected in practically all their functions. In mammals, the main structure that forms this circadian biological clock is the suprachiasmatic nucleus (SCN) located in the hypothalamic region of the brain. SCN has the particularity of producing spontaneous electric changes through the day and transmitting them to the other structures of the organism. Thus, every body function has a circadian rhythm. Our body has the capability of “setting” the times of day, or, in other words, each body function has an optimum moment at a specific time of day. The most evident circadian rhythm is the alternation between sleep and wakefulness: a good night sleep is essential to feel awake during the day. But this is, by no means, the only rhythm. For example, the temperature of the body lowers during the night and from the next morning on, it starts slowly climbing until the end of the afternoon, when it reaches its maximum temperature. Hormone secretions also have a circadian rhythm; the hormone cortisol, for example, rises in the early hours of the morning and stays in the