Special Issue on “Research on Circadian Rhythms in Health and Disease”

Despite rigorous investigation of circadian rhythms in humans and animal models in the past, basic chronobiologic principles have not yet entered clinical practice. Advances in animal models and in human physiological monitoring as well as recent developments of applied techniques in biomedical research, including unobtrusive wearables, now facilitate the incorporation of chronobiologic principles and their transfer into clinical practice for timed screening, diagnosis, and treatment. While technological advances focus primarily on data collection, less attention has been devoted to the processing and analysis of the new large databases thus acquired. It is imperative that measures be taken at this stage to include all information needed to foster the development of true precision chronomedicine. For this purpose, it is crucial to establish universal standards for the collection, analysis, and interpretation of time series data to help discern rhythms in health from their altered characteristics in disease. It is equally important that data banks and repositories suitable for time series analysis also specify age and gender, as well as genetic information whenever possible.

This Special Issue includes six original research papers and two reviews discussing various aspects of applied research on circadian rhythms in health and disease. As a major signal that modulates circadian rhythms in mammals, light holds a prominent place in this issue. Arguments are provided to illustrate that tools are needed for studying and evaluating real environments from both visual and nonvisual perspectives to improve lighting conditions in accordance with current standards. Sanchez-Cano et al. emphasize the need to personalize optimal lighting specifications [1]. They suggest a personalized method to calculate melanopic light reaching the retina depending on the optical density of the crystalline lens. Its importance lies in the fact that aging affects the lens structure and modifies light wavelength absorption, causing significant variation in the level of daylight equivalent melanopic illuminance in the retina. Otsuka et al. [2] derive rules of heliogeomagnetic coordination of circadian rhythms in blood pressure and heart rate, with illustrations based on data collected above the Arctic Circle. They show that mild, moderate, or extreme geomagnetic activity induce differential changes in circadian parameters (amplitude, acrophase, and period) according to a bell-shaped windowed dose–response curve depending on the intensity of geomagnetic disturbances and on the circadian phase when geomagnetic stimulation occurs. Putilov et al. [3] suggest a modeling technology for the correction of accumulated sleep loss during weekly workdays. The model takes into account individual differences between morning and evening chronotypes (M-types and E-types), suggesting that a larger-than-2-hour difference between M- and E-types in weekday waking times is necessary to equalize losses between these types. Borisenkov et al. [4] perform a comparative analysis of actigraphic parametric and non-parametric indices regarding their agreement with the MCTQ (Munich ChronoType Questionnaire) and the PSQI (Pittsburgh Sleep Quality Index) questionnaire. They determine that non-parametric indices are better predictors than parametric ones, but find the acrophase of activity to be a useful index to predict sleep efficacy. Gubin et al. [5] document that compromised light perception, due to retinal ganglion cell loss, is associated with altered serum lipid profiles, showing uneven alterations in the morning or evening values, also dependent on CLOCK gene polymorphism. Their discovery underlines a putative link between light signaling and lipid metabolism, and highlights the importance of analyzing evening as well as morning values, suggesting the morning–evening gradient as a novel biomarker. The association of sleep disturbances with co-morbid psychological mental problems is investigated by Rahafar et al. [6], who provide insight into the interaction between sleep habits of individuals regarding their Dark Triad traits (Machiavellianism, psychopathy, and narcissism). Weinert and Gubin [7] review the effect of physical activity on the circadian system, and outline its benefits for health, well-being and performance. The authors propagate practical implications of circadian activity rhythms for diagnosis and highlight the advantages of scheduled motor activity. Finally, Danilenko et al. [8] provide a comparative review of seventeen currently available light-and-motion wearables for their appearance, dimensions, weight, mounting, battery, sensors, features, communication interface, and software. This review is a useful guide helping researchers to make a proper choice to suit their needs for a particular research protocol.

Contributions in this Special Issue of Applied Sciences cover important developments in the field of chronobiology and offer a vision for the future of the field. They will interest students in life sciences as well as seasoned researchers and all those involved in the development of wearable sensors and devices, including the software architecture needed for the analysis and interpretation of the serial data. A concerted collaborative effort between chronobiologists and engineers should lead the way toward a true precision chronomedicine that urgently needs entering everyday clinical practice.