ear Colleagues,Sleep is a part of the physiological sleep-wake cycle and constitutes an essentialchronobiological marker in human life.The sleep-wake cycle can be modified by: Environmental conditions (social rhythms, professional cues, and noise, light, and tem-perature variations). Phenomena affecting the individual:- Psychiatric disorders (depression, anxiety, and situations of dependence or rebound).- Organic sleep disorders (sleep apnea, periodic limb movement disorders, and narcolepsy).- Somatic disorders (acute or chronic pain, diabetes, side effects of other treatments, andinfectious or parasitic diseases).In addition to being a chronobiological marker, sleep can also be considered as amarker of a change in state, and whether or not a medical treatment modifies the basal stateis an important feature of its therapeutic profile.We can therefore characterize the qualitative aspects of a treatment, quantify its ther-apeutic effects, detect some of its side effects, and determine the length of treatmentrequired, according to the results of sleep studies.This is why such studies are so importantto research and development not only in the field of psychotropic drugs, but also for somaticdrugs with peripheral targets but central effects.Together with Prof Michael Davidson, we have tried to document certain aspects ofthe above in this issue of Dialogues in Clinical Neuroscience.We would like to thank ProfDavidson and the authors who agreed to participate in this issue, Profs C. Guilleminault, O.Le Bon,A. Muzet, L. Staner, J. Haba-Rubio,A. Buguet, and Y. Dagan.Yours sincerely,Jean-Paul Macher, MDMarc-Antoine Crocq, MDE d i t o r i a lD2 8 5