Background: Dexmedetomidine, a potent α-2-adrenergic agonist, is widely used as sedative in critically ill patients. This pilot study was designed to assess the effect of dexmedetomidine administration on sleep quality in critically ill patients. Methods: Polysomnography was performed on hemodynamically stable critically ill patients for 57 consecutive hours, divided into three night-time (9:00 pm to 6:00 am) and two daytime (6:00 am to 9:00 pm) periods. On the second night, dexmedetomidine was given by a continuous infusion targeting a sedation level −1 to −2 on the Richmond Agitation Sedation Scale. Other sedatives were not permitted. Results: Thirteen patients were studied. Dexmedetomidine was given in a dose of 0.6 μg kg⁻¹ h⁻¹ (0.4 to 0.7) (median [interquartile range]). Compared to first and third nights (without dexmedetomidine), sleep efficiency was significantly higher during the second night (first: 9.7% [1.6 to 45.1], second: 64.8% [51.4 to 79.9], third: 6.9% [0.0 to 17.1], P < 0.002). Without dexmedetomidine, night-time sleep fragmentation index (7.6 events per hour [4.8 to 14.2]) and stage 1 of sleep (48.0% [30.1 to 66.4]) were significantly higher (P = 0.023 and P = 0.006, respectively), and stage 2 (47.0% [27.5 to 61.2]) showed values lower (P = 0.006) than the corresponding values (2.7 events per hour [1.6 to 4.9], 13.1% [6.2 to 23.6], 80.2% [68.9 to 92.8]) observed with dexmedetomidine. Without sedation, sleep was equally distributed between day and night, a pattern that was modified significantly (P = 0.032) by night-time dexmedetomidine infusion, with more than three quarters of sleep occurring during the night (79% [66 to 87]). Conclusion: In highly selected critically ill patients, dexmedetomidine infusion during the night to achieve light sedation improves sleep by increasing sleep efficiency and stage 2 and modifies the 24-h sleep pattern by shifting sleep mainly to the night.