Melatonin is an endogenous neurohormone that is produced in most living organisms, including unicellular and multicellular organisms, plants, vertebrates, and nonvertebrate animals. In diurnal animals, endogenous melatonin functions as a neurohormone and contributes to circadian rhythms. In nocturnal animals, endogenous melatonin no longer functions as a contributor to circadian rhythms. Circadian rhythms control the timing, quantity, and quality of hormones and neurotransmitters that the body produces and eventually secretes. An imbalance of these events creates disturbances in circadian rhythm. During disturbances of circadian rhythm, the body produces hormones, chemicals, and neurotransmitters in aberrant amounts or at the wrong time of day. The human circadian system is synchronized with physiological functions and metabolism. Many studies have reported that exogenous melatonin has analgesic and neuroprotective effects in chronic pain. Considering that chronotherapy may be beneficial for the treatment of chronic pain, the present review describes the properties, possible mechanisms, and function of melatonin in chronic pain.
Keywords: Central poststroke pain; Chronic pain; Chronotherapy; Circadian rhythms; Melatonin; Neuroprotection.