Sleep is a fundamental biological process that is as important to survival as are eating and breathing. We now know that adequate sleep is essential for physical and mental health. Yet, the precise functions of sleep – what sleep does for the brain and body – remain elusive. The relationship among cancer, sleep disruption, and sleep disorders is discussed inChapter 14. The focus of this chapter is basic sleep mechanisms that can lead to cancer-related sleep disturbances, particularly those mechanisms that may be susceptible to perturbation during pathology.

Sleep consists of two major phases, rapid-eye-movement (REM) sleep and non-REM (NREM) sleep. In human sleep research and in sleep-disorders medicine, the NREM sleep phase is further subdivided into three stages that may generally be considered to parallel a continuum of sleep depth: stage N1 is the light sleep that follows wakefulness; during stage N2, two electroencephalogram (EEG) features, spindles and K-complexes, are typically found; stage N3 is referred to as slow-wave sleep and is characterized by a preponderance of high-amplitude, low-frequency EEG components. Preclinical sleep researchers generally find it sufficient to define only the two sleep phases, REM sleep and NREM sleep, in laboratory research animals.

Healthy sleep consists of cycles of orderly transitions from wakefulness to NREM sleep to REM sleep. Healthy human adults during the course of a night's sleep will have four to six NREM–REM cycles of about 80–110 minutes duration, whereas rodents will have many NREM–REM cycles, each lasting about 8–10 minutes.