The regular alternation of light and dark affects not only human biological systems, but also the social organization of behavior. The effect of such light modes is manifested in periodic changes in physiological functions and biological rhythms exhibited at every level of life. The book discusses some of the specificities of the circadian rhythms in living organisms and mentions aspects of the control of circadian rhythms as well as experimental and clinical cases that are closely related to circadian disruption. This book can evoke interest in many researchers who want to use this information for the advancement of their research towards a better understanding of the biological time structure.