This study described the sleep disturbances of 115 lung cancer patients undergoing their fourth cycle of chemotherapy and examined the impact of sleep disturbances on quality of life and functional performance status while controlling for pain, depression, fatigue, and dyspnea. Sleep disturbance and quality of life were assessed by the Pittsburg Sleep Quality Index (PSQI), and European Organization for Research and Treatment Quality of Life Questionnaire-Cancer 30 (EORTC), respectively. Data were also collected on covariates of sleep disturbance: performance status, pain, fatigue, depression and dyspnea. Patients' mean PSQI global scores for days with chemotherapy (6.86+/-3.83) and for days without chemotherapy (6.23+/-3.47) were both higher than the cut-off of 5, indicating poor quality of sleep during the fourth cycle of chemotherapy. After controlling for covariates, sleep disturbance was significantly associated with impaired cognitive function (EORTC) and poorer functional status. Our results suggest that clinicians should routinely assess sleep problems in lung cancer patients.