Co-morbid insomnia is a much more frequent problem than primary insomnia. In co-morbid insomnia, management of the underlying disease can improve sleep difficulty. Conversely, treating the sleep disorder may also improve the co-morbid condition. For example, patients with painful chronic illnesses are more likely to experience sleep disturbance than patients with non-painful illnesses. Moreover, there is evidence that insomnia further exacerbates pain in these illnesses. This suggests that a reciprocal relationship exists between pain and sleep, and that intervention targeted primarily at insomnia may improve pain. Treatment options for sleep disorders in the context of pain that have been assessed include cognitive behavioural therapy for insomnia and various pharmacological therapies. In randomized clinical trials, cognitive behavioural therapy significantly improved insomnia secondary to chronic pain compared with control therapy, but pain was only improved in patients in whom it was associated with pain disorders other than fibromyalgia. Of the pharmacological agents studied (zopiclone, zolpidem and triazolam), only triazolam improved both sleep and pain to a greater extent than placebo. Overall, clinical data supporting a cause-effect relationship between insomnia and pain are preliminary and are limited to several small trials. Further investigation is required to clarify the extent of the link between insomnia and pain and whether successfully managing insomnia secondary to pain improves pain symptoms. Areas of particular interest include investigation of the effect of sleep agents on analgesia and the effect of analgesics on sleep.