Environmental lightmeasurements and effects on the human circadian system.

Abstract

The main objective of this Doctoral Thesis was to investigate the effects of light, both natural and artificial, on the human circadian system. The aim of chapter 1 was to improve the ambulatory circadian monitoring device (ACM, Kronowise¿) capabilities by developing an algorithm that allows to determine light intensity, timing and circadian light stimulation by differentiating between full visible, infrared and circadian light, as well as to discriminate between different light sources (natural and artificial with low and high infrared composition) in subjects under free living conditions. Our results show that the ACM device provided with three light sensors and the algorithm developed here allow an accurate detection of light type, intensity and timing for full visible and circadian light, with simultaneous monitoring of several circadian marker rhythms that will open the possibility to explore light synchronization in population groups while they maintain their normal lifestyle. The main objective of chapter 2 was to determine the influence of light color temperature on two circadian markers: melatonin inhibition and pupillary light reflex in young and aged people. Furthermore, we aimed to investigate the best theoretical model that fits its prediction to our results. We confirmed that even the light condition with less content in blue radiation produced significant melatonin suppression. On the other hand, higher content in blue spectrum produced higher contraction in the PIPR regulated by ipRGCs. In conclusion, both PLR and melatonin suppression can be useful techniques to analyze light impact on circadian system since they provide different and complementary information. The objective of chapter 3 was to develop a practical method to evaluate artificial light sources, based not only on their spectral power and thus, the corresponding activation of circadian photoreceptors, but also chromatic discrimination, comfort and circadian effects. Our results proved the method here proposed is an adequate tool to evaluate the effects of any light source on the circadian system, since it focuses the assessment from three different approaches that are complementary and interdependent and considers both objective and subjective variables. Considering that DST transitions modify human behavior and, therefore, their lighting exposure patterns, the aim of chapter 3 was to investigate that relationship considering also sleep and the human phase response curve to light. These results show that during both DST transitions, the morning light exposure (in the phase advance zone) is the most influenced by the new local time, probably because during the afternoon and night, the use of artificial lighting is predominant regardless the time shift. Up to our knowledge, this is the first study monitoring personal light exposure in participants under natural conditions before and after DST transitions. Our results reflects that March DST transition acts as a phase advance and adaptation to it is more difficult than to October transition, since it represents a phase delay, which is the natural tendency of circadian central pacemaker. Moreover, our findings reflect the great influence of artificial lighting on phase shifts. Both natural and artificial lighting have an effect on the human circadian system, and this latter constitutes an important variable to consider. Kronowise¿ light sensors and our algorithm are useful tools to assess these effects under free living conditions and in chronodisruptive situations, such as daylight saving time transitions. The development of integrative and non-invasive protocols and measurements will help us to deep in understanding of circadian adjustment and mechanisms involved in chronodisruption.