Biological rhythmicity has been a subject of scientific research for a relatively short time. In the special case of daily, or circadian rhythms, it is only during the past twenty years that rapidly increasing efforts have been undertaken in evaluat­ ing properties and mechanisms. As a consequence of these efforts, the study of biological and, in particular, circadian rhythmicity is no longer a somewhat dubious occupation but rather a serious branch of science which combines the interdisciplinary efforts of numerous researchers around the world. The general result of these efforts is that many features of circadian rhythms of many different species of living beings are well known today. In addition to studies with lower organisms, the evaluation of human circadian rhythms was originally more or less a compulsory exercise done in order to extend the "catalogue of species"; of course, the work was of unusual impor­ tance due to the special position of man in biology. In the course of the very first experimental series, it became clear that humans possess an "internal clock" as had been established in various organisms, protists, plants, and animals, and that human circadian rhythms fit the general regularities of biological rhythms known at that time. However, it soon became apparent that circadian rhythmicity of man shows, additionally, particularities of great general interest, for practical and theoretical reasons.

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1 Introduction.- 1.1. The Circadian System.- 1.2. Human Circadian Rhythms.- 1.3. Operations and Methods.- 1.3.1. Facilities and Subjects.- 1.3.2. The Measurements.- 1.3.3. The Analyses.- 2 Autonomous Rhythms.- 2.1. External Synchronization and Desynchronization.- 2.1.1. Free Running Rhythms.- 2.1.2. Criteria for Autonomy.- 2.1.3. Criteria for Oscillatory Origin.- 2.1.4. Application of the Criteria to Critical Experiments.- 2.1.5. Summary of Free Running Rhythms.- 2.2. Internal Synchronization and Desynchronization.- 2.2.1. Real Internal Desynchronization.- 2.2.2. Apparent Internal Desynchronization.- 2.2.3. Internal Dissociation.- 2.2.4. Internal Desynchronization: Other Indications.- 2.2.5. Conclusions.- 2.3. Review of Autonomous Rhythms.- 2.4. External Modifications of Autonomous Rhythms.- 2.4.1. Influence of Light Intensity.- 2.4.2. Influence of Light Modality.- 2.4.3. Influence of Ambient Temperature.- 2.4.4. Influence of Natural Electromagnetic Fields.- 2.4.5. Influence of Artificial Electromagnetic Fields.- 2.4.6. Influence of Physical Workload.- 2.4.7. Influence of Psychical Burden.- 2.4.8. Influence of Social Contacts.- 2.4.9. Summary.- 3 Heteronomous Rhythms.- 3.1. General Effects of External Zeitgebers.- 3.1.1. Alterations of the Zeitgeber Period.- 3.1.2. Shifts of the Zeitgeber Phase.- 3.1.3. Entrained versus Free Running Rhythms.- 3.2. Modalities of Effective Zeitgebers.- 3.2.1. Light-dark Zeitgebers.- 3.2.2. Physical versus Social Zeitgebers.- 3.2.3. Electric Field Zeitgebers.- 3.2.4. Summary.- 3.3. Partial Synchronization.- 3.3.1. Strong Light-dark Zeitgebers.- 3.3.2. External Modifications of Partially Desynchronized Rhythms.- 3.3.3. Experiments with Continuously Changing Zeitgeber Period.- 3.3.4. Review of Partially Synchronized Rhythms.- 3.4. General Effectiveness of Light-dark Zeitgebers.- 3.5. General Mode of Zeitgeber Action.- 4 Synthesis.- 4.1. The One Oscillator Model.- 4.1.1. Autonomous Rhythms.- 4.1.2. Entrained Rhythms.- 4.1.3. Conclusions.- 4.2. The Multioscillator Model.- 4.2.1. Internal Dissociation.- 4.2.2. Internal Desynchronization.- 4.2.3. Properties of Separated Oscillators.- 4.3. Internal Interaction Between Different Rhythms.- 4.3.1. Oscillatory Interaction.- 4.3.2. Reactive Interactions.- 4.3.3. Separation of the Interactions.- 5 Conclusions and Speculations.- 5.1. Theoretical Aspects.- 5.2. Practical Aspects.- 5.2.1. Problems of Phase Maps.- 5.2.2. Problems of Rhythm Disorders.- 6 References.- Author Index.