Sleep deprivation is common in intensive care unit (ICU) patients. The criterion standard for sleep monitoring, polysomnography, is impractical in ICU. Actigraphy (a wrist watch indicating amount of sleep) proved to be a good alternative in non-ICU patients, but not in prolonged mechanically ventilated patients, probably due to ICU-acquired weakness. Short-stay ICU patients do not suffer from ICU-acquired weakness. However, the accuracy of actigraphy is unknown in these patients. Therefore, we compared actigraphy to polysomnography in short-stay ICU patients. Sleep measurements were conducted in 7 postcardiothoracic surgery patients. The sensitivity (percentage of actigraphy data that agreed with sleep determined using polysomnography) and specificity (percentage of actigraphy data that agreed with awake determined using polysomnography) were calculated. The result showed that actigraphy underestimated the amount of wake time and overestimated the amount of sleep. The median specificity for actigraphy was always less than 19% and sensitivity more than 94%. Therefore, actigraphy is not reliable for sleep monitoring in short-stay ICU patients.