Individuals with spinal cord injury (SCI) commonly complain about difficulty in sleeping. Although various sleep disordered breathing definitions and indices are used that make comparisons between studies difficult, it seems evident that the frequency of sleep disorders is higher in individuals with SCI, especially with regard to obstructive sleep apnea. In addition, there is a correlation between the incidence of sleep disturbances and the spinal cord level injured, age, body mass index, neck circumference, abdominal girth, and use of sedating medications. Regulation of respiration is dependent on wakefulness and sleep. Thus, it is important to be aware of basic mechanisms in the regulation and control of sleep and awake states. Supine position decreases the vital capacity in tetraplegic individuals, and diminished responsiveness to Pa(CO)(2) may further decrease ventilatory reserve. There also may be a potential disparity between daytime and nocturnal ventilation, as individuals with partially reduced muscle tone are susceptible to not only sleep apnea, but also sleep-related hypoventilation which may be aggravated during rapid eye movement sleep.