Possible role of the opioid peptides in sleep.

The opioid peptide endorphins, enkephalins, and dynorphins--found in brain, pituitary, and gut--are neurohormones involved in the regulation of a number of seemingly diverse biologic activities, including respiration, mood, pain perception, blood pressure, body temperature, and certain visceral responses. When viewed in integrated fashion, however, the spectrum of activities induced by the administration of both the exogenous opiates (e.g. morphine) and the endogenous opioids resembles a natural physiologic state: the sleep state. We propose that the opioid peptides in conjunction with the peptide neurohormone vasopressin are involved in the induction and maintenance of the sleep state. We also propose that the function of sleep is to protect an animal during periods when it is at a selective disadvantage, and we provide evidence to support and integrate both concepts.