Fatigue is a major complaint among cancer patients, yet it is unknown whether cancer-related fatigue experienced during the day relates to sleep/wake cycles or to the quality and quantity of sleep obtained at night. Although it is not well defined or well understood at present, cancer-related fatigue is generally regarded as a form of tiredness that does not improve following rest or sleep. Objectively recorded sleep and biological rhythms have not been well investigated in these patients, but it appears that most cancer patients may in fact not be getting a good night's sleep. Evidence is accumulating that sleep is often disturbed in cancer patients, probably owing to a variety of causes. We posit that some degree of cancer-related fatigue experienced during the day may relate to sleep/wake cycles or to the quality and quantity of sleep obtained at night. Different components or dimensions of fatigue (physical, attentional/cognitive, emotional/affective, etc.) are probably associated in some way with disrupted sleep and desynchronized sleep/wake rhythms. These associations may change in measurable ways prior to treatment, during treatment and after treatment completion. In cancer patients, as in other medically ill patients, sleep that is inadequate or unrefreshing may be important not only to the expression of fatigue, but to the patients' quality of life and their tolerance to treatment, and may influence the development of mood disorders and clinical depression. This review summarizes the state of the literature on fatigue, sleep and circadian rhythms.