Melatonin and the Inter-­‐relationship between Sleep, Pain and Mood An Observational Study

Abstract

Sleep is a biological phenomenon that all humans exhibit, emphasizing its essential function in regulating normal physiology. Therefore, it is not surprising that sleep dysfunction is a comorbidity associated to chronic pain and psychiatric disorders. A vicious cycle exists between sleep, chronic pain and mood disorders where poor sleep exacerbates pain and mood symptoms, which, in turn, worsens sleep. Melatonin, the endogenous sleep hormone, may be mediating this inter-relationship as it is independently associated with each of the three factors, showing hypnotic, analgesic and antidepressant properties. Understanding the mechanisms behind this inter-relationship will help better treatment regimens to concurrently treat the co-morbidities. This current study aimed to further delineate the inter-relationships between sleep, pain and mood, focusing on subclinical aspects of acute pain and mood. It was hypothesized that melatonin is regulating the inter-relationships between the three factors. 23 insomniasufferers and 22 healthy sleepers provided subjective, diary-derived, and objective actigraphy-derived sleep measures, accompanied with daily mood reports, over a week prior to an acute noxious stressor, exerted by a cold-pressor test (CPT). CPT measured the severity of pain experienced. Nocturnal melatonin production on the night before the CPT was detected. Insomniacs showed lower melatonin production and an overall poorer sleep quality, pain sensitivity and mood negativity, than controls. However, melatonin was only significantly correlated to objective sleep measures. Significant inter-relationships (p<0.05) existed between the subjective reports of sleep, acute pain and mood, but endogenous melatonin showed negligible effects on the power of these inter-relationships. The correlation between subjective variables suggests that cognitive factors influence the inter-relationships. Therefore, insomnia results in worse health outcomes, with respect to pain and mood, but the beneficial health effects associated with melatonin may only be an epiphenomenon to improvements in sleep quality.