The field of circadian rhythm disorders gains recognition within the sleep medicine, although it is true to say that this field is neither well understood nor fully utilised. The example for the later is the field of chronopharmacology, which promises to improve treatment outcome for a number of disorders, including those difficult to treat such as cancer. Another example of successful implementation of findings from the field of circadian rhythms is that of delayed morning school time endorsed by a number of school boards, which often results in improved rate of absenteeism and in the better academic achievements. At the level of a medical profession, better understanding of fundamental principles of circadian rhythms can lead to better and more efficient treatment, with the shift of a therapeutic approach from medications to implementation of cognitive-behavioural measures, and to, ultimately, better rapport with these patients. The article deals with some fundamental issues in respect to chronobiology, circadian rhythms and circadian rhythm disorders.