The exact effect of opioid analgesics on sleep is to be determined. Although literature data are sporadically reported, the aim of this review is to summarize the already known effects of such medications on sleep. A variety of effects, both positive and negative, has been suggested, when opioids are used for pain treatment, but in the absence of pain as well. Although often thought to promote restful sleep, the reality is much more complicated. Sleep disturbances and alterations of sleep quantity and quality have been reported. In addition, their sedative effects have been relatively well established and opioids can cause respiration to slow and become irregular, leading to hypercapnia and hypoxia. As a result, their usage has been linked to irregular or ataxic breathing (Biot's breathing) and their use has been associated with both central and obstructive sleep apnea. One could estimate that central apnea is a common complication of such chronic therapy, affecting between 30% and 90% of patients. Thus, sleep disturbances can be induced or deteriorated. On the other hand, extended release opioid formulations have been suggested to improve sleep due to no analgesic gaps and less walking because of breakthrough pain. Furthermore, several reports have shown significantly improved sleep quantity and adequacy, with reduced sleep disturbances. Still, as no prospective trials on the effect of opioid therapy on sleep are available and evidence is scarce, definitive conclusions cannot be drawn. Future studies with their effect on sleep as primary end-point are needed to draw permanent conclusions.