Objectives: Delirium is a common and serious complication among hospitalized older adults. Although sleep and circadian disturbances are common in delirium, evidence on changes preceding onset remains limited.This study used actigraphy to objectively assess sleep and circadian parameters in older adults admitted to medical wards and to identify early actigraphy-derived markers associated with delirium. Methods: This prospective observational study included hospitalized adults aged ≥60 years admitted to medical wards. Delirium was evaluated twice daily using the 4-A’s Test (4AT). sleep and circadian parameters were continuously recorded for up to 7 consecutive days following admission , with analyses restricted to data collected prior to delirium onset. Factors associated with delirium were determined using multivariable Cox proportional hazards regression. Results: Among 257 participants, 59 (22.96%) developed delirium. Patients who developed delirium were older than those who did not (81.83 vs. 76.33 years). In multivariable Cox regression analysis, high malnutrition risk (HR = 2.57, 95% CI: 1.20–5.52), lower MMSE scores (HR = 0.92, 95% CI: 0.87–0.96), lower serum sodium levels (HR = 0.94, 95% CI: 0.89–0.99), elevated neutrophil-to-lymphocyte ratio (HR = 2.82, 95% CI: 1.01–7.91), and reduced sleep efficiency (HR = 0.96, 95% CI: 0.93–0.99) were independently associated with subsequent delirium. Conclusions: Delirium was common among hospitalized older adults and was associated with clinical, laboratory, and sleep-related factors, with reduced sleep efficiency emerging as an independent predictor. Actigraphy-derived sleep and circadian parameters may serve as early markers to facilitate timely detection and prevention of delirium.