Chronobiology and mood disorders: background and introduction.

The papers in this special section are the proceedings of a symposium on chronobiology and mood disorders presented at the June 1999 meeting of the Canadian College of Neuropsychopharmacology in Halifax, Nova Scotia. These papers review aspects of mood disorders that are linked to biological rhythms with 3 different periodicities: daily, menstrual and annual. They also reflect a growing interest in mechanisms regulating physiological rhythmicity on a variety of time scales and in the impact of normal and pathologic function of these mechanisms on psychiatric illnesses. Rhythms are related to mood disorders in several different ways. First, some conditions are expressed cyclically; that is, they have a period linked to an identified internal or external periodicity, such as menstrual and annual rhythms of dysphoria or depression. Other conditions, such as many cases of bipolar illness, express cycles that are not tied to any identified periodic stimulus. A second connection between rhythms and mood disorders is the alteration of daily rhythms, in particular sleep and waking, in mood disorders. Finally, the systems responsible for the generation or synchronization of normal rhythms may be involved in the etiology of mood disorders, as has sometimes been proposed, for example, for bipolar illness and for seasonal affective disorders. Connections between psychiatric disorders and biological rhythms were reviewed more than 40 years ago in a landmark monograph by Curt Paul Richter, whose innovative concepts and studies gave birth to many fields of psychobiological research, as well as establishing key concepts and analytic methods for the study of biological rhythms. Richter’s 1965 monograph Biological Clocks in Medicine and Psychiatry presented material from two 1959 Salmon Lectures to the New York Academy of Medicine. It serves as a benchmark for assessing the developments in this area of research over the last few decades.