Study Objectives: To describe preoperative and postoperative sleep disruption and its relationship to postoperative delirium. Design: Prospective cohort study with 6 time points (3 nights pre-hospitalization and 3 nights post-surgery). Setting: University medical center. Patients: The sample consisted of 50 English-speaking patients ≥ 40 years of age scheduled for major non-cardiac surgery, with an anticipated hospital stay ≥ 3 days. Interventions: None. Measurements and results: Sleep was measured before and after surgery for a total of 6 days using a wrist actigraph to quantify movement in a continuous fashion. Postoperative delirium was measured by a structured interview using the Confusion Assessment Method. Sleep variables for patients with (n = 7) and without (n = 43) postoperative delirium were compared using the unpaired Student t-tests or χ 2 tests. Repeated measures analysis of variance for the 6 days was used to examine within-subject changes over time and between group differences. The mean age of the patients was 66 ± 11 years (range 43-91 years), and it was not associated with sleep variables or postoperative delirium. The incidence of postoperative delirium observed during any of the 3 postoperative days was 14%. For the 7 patients who subsequently developed postoperative delirium, wake after sleep onset (WASO) as a percentage of total sleep time was signifi cantly higher (44% ± 22%) during the night before surgery compared to the patients who did not subsequently developed delirium (21% ± 20%, p = 0.012). This sleep disruption continued postoperatively, and to a greater extent, for the fi rst 2 nights after surgery. Patients with WASO < 10% did not experience postoperative delirium. Self-reported sleep disturbance did not differ between patients with vs. without postoperative delirium. D elirium is a major challenge facing geriatric practice due to its prevalence, complex etiology, and potential severe impact on patients. Postoperative delirium is associated with longer hospital stays, worse functional outcomes, higher healthcare costs, and increased mortality.1 Delirium develops through a complex interaction between the patient's baseline vulnerability (predisposing risk factors before hospitalization) and precipitating factors or insults (events that occur during hospitalization). Some of the vulnerability factors identifi ed include advanced age, cognitive impairment or dementia, and preexisting comorbidities.2,3 Over the last two decades, a large body of literature has focused on the clinical manifestations, risk factors and outcomes of postoperative delirium. Sleep disruption, in particular, has frequently been cited as an important etiological factor associated with the development of delirium. 4 Sleep disturbance, especially sleep fragmentation, and poor sleep quality are commonly observed in older adults. In the hospital, environmental factors and health care practices further contribute to sleep disruption. BRIEF SUMMARYCurrent Knowledge/Study Rationale: Sleep disruption has frequently ...