Day–night contrast as source of health for humans

Abstract

Introduction Modern lifestyle is mainly characterized by no environmental differences between day and night, either in light exposure (artificial light) or in activity (sedentarism), or environmental temperature or feeding (frequent snacking), and as a consequence, an impaired circadian system (CS) through a process known as chronodisruption appears. This process is related to human pathologies such as certain cancers, metabolic syndrome, and affective and cognitive disorders. However, little is known about CS enhancement. The aim of this work is to propose increased day/night contrast as a strategy for chronoenhancement. Materials and methods Light exposure, environmental temperature, Wrist Temperature (WT), activity, body position and sleep were recorded under free-living conditions during one week young volunteers (n = 131). Subjects were classified according to contrast in each variable. High contrast (HC) or low contrast (LC) for each variable were selected to analyze the effect of light exposure, environmental temperature, activity, body position, and sleep day/night differences would have on WT and to obtain the circadian pattern for all this variables that corresponds to HC or LC for WT. Results HC in lifestyle variables showed better rhythms than LC subjects except sleep that it is higher stability and lower fragmentation for WT. Subjects with HC and LC for WT also demonstrated differences in lifestyle, where HC subjects showed a slightly advanced night phase onset and a general increase in day/night contrast. Besides, simulated high day/night contrast by mathematical models suggests an improvement in WT parameters by increasing lifestyle contrast. Finally, some individuals were classified as belonging to the HC group in terms of WT when they were exposed to the lifestyle characteristic of the LC group, while others exhibit WT arrhythmicity despite their good lifestyle habits, revealing two different WT components: an exogenous one modified by lifestyle and another endogenous component that is refractory to it. Conclusion Although some subjects could be refractory to cronoenhancement, potentiating day/night contrast in subject’s lifestyle has proven to be a feasible measure to attenuate chronodisruption in modern societies. Acknowledgements Study supported by RETICEF (RD12/0043/0011), MINECO (BFU2010–21945-C02–01), and INNPACTO (IPT-2011–0833-900000) with FEDER cofounding to JAM, and a research fellowship to Antonio Martinez-Nicolas (University of Murcia).