Abstract: Biological clock existing among most organisms is close related to the physiological and behavioral events. In particular, the regulation of biological clock is connected with the sleep mechanism of human beings. This course provides the fundamental concepts of biological clock as well as the main issues on common sleep disorder. Studying on biological clock approaches the relationship between biological phenomena and time. Besides, the mechanisms and regulations of biological clock system in the organisms are examined as well. This course starts to discuss the universal of biological clock and the meaning of presence among most organisms. Then it moves to the regulatory mechanism of biological clock and localization of clocks in different organisms such as microorganisms, plants and animals. We also introduce the circadian clock which regulates physiological and behavioral events in a day using some vivid examples. As the later half part of the course, it would be focused on the sleep physiology and hygiene of human. Sleep mechanism is controlled by circadian clock. Some sleep disorders are caused by the problem of circadian clock system. We are going to introduce some common sleep disorders such as parasomnia, insonimia, etc. In the end of course, we hope students know the importance of sleep and how to manage their sleep.