Sleep disorders and insomnia are more prevalent in patients with cancer than in the normal population. Sleep disorders consist of delayed sleep latency, waking episodes after sleep onset, unrefreshing sleep, reduced quality of sleep, and reduced sleep efficiency. Sleep disorders cluster with pain, fatigue, depression, anxiety, and vasomotor symptoms, depending on stage of disease, treatment, and comorbidities. Premorbid sleep problems and shift work have been associated with a higher prevalence of cancer; in fact, shift work has been labeled a carcinogen. Treatment for insomnia includes cognitive behavioral therapy with sleep hygiene, bright-light therapy, exercise, yoga, melatonin, and hypnotic medications. Unfortunately, there are few randomized trials in cancer-related sleep disorders such that most recommendations particularly for hypnotics are based on treatment for primary insomnia. In this article, insomnia is reviewed as a predisposing factor to cancer, prior to and during treatment, in cancer survivorship and in advanced cancer. Recommendations for treatment are based on low-quality evidence but are also reviewed.