**1. Introduction: basics of sleep-pain interaction**

Sleep and pain interaction occur at several levels, both in physiology and pathology, and is influenced by a circadian timing system from which almost all functions in our body partially depend. Sleep and pain are both vital functions that ultimately contribute to general body homeostasis, and therefore to life success. Experience of pain motivates an individual to run away from potential physical injury and/or to stay protected and immobilized if an injury already occurred in order to promote and optimize recovery. While the escape-related response is tied to an increased state of arousal (being more awake), the response related to protection is linked to a state of rest (and sleep). Interestingly, sufficient sleep of good quality and pain have an inverse epidemiological relationship over the lifespan: sleep reduction, either in terms of duration or quality, increases from youth to old age, and at the same time pain-related complains follow a similar pattern. This relationship is clinically important since such negative interaction disturbs the internal milieu, and therefore impacts on prognosis of many disorders co-occurring with one or both of the symptoms of disturbed sleep and pain. Some of the most common pain-related conditions occur in the oro-maxillo-facial complex and are associated with emotional, psychological, and social disturbances that seriously compromise the patient's quality of life. Thus, orofacial pain may by itself directly and indirectly interfere with sleep quality and sleep duration and consequently increases the severity of concomitant conditions and even the pain-related outcomes. On the other hand, insufficient or inadequate sleep is known to contribute

to an increased pain intensity and a reduction of pain tolerance. A vicious cycle can then be perpetuated, and therefore an adequate knowledge on the sleep-pain interaction-related mechanisms should be an important part of learning and training in the domain of clinical sleep neurology, which is in the scope of this book.
