The relationship between tiredness and the risk of medical errors is now commonly accepted. The main objective of this study was to assess the impact of an intensive care unit (ICU) night shift on the cognitive performance of a group of intensivists. The influence of professional experience and the amount of sleep on cognitive performance was also investigated. A total of 51 intensivists from three ICUs (24 seniors and 27 residents) were included. The study participants were evaluated after a night of rest and after a night shift according to a randomized order. Four cognitive skills were tested according to the Wechsler Adult Intelligence Scale and the Wisconsin Card Sorting Test. All cognitive abilities worsened after a night shift: working memory capacity (11.3 +/- A 0.3 vs. 9.4 +/- A 0.3; p < 0.001), speed of processing information (13.5 +/- A 0.4 vs. 10.9 +/- A 0.3; p < 0.001), perceptual reasoning (10.6 +/- A 0.3 vs. 9.3 +/- A 0.3; p < 0.002), and cognitive flexibility (41.2 +/- A 1.2 vs. 44.2 +/- A 1.3; p = 0.063). There was no significant difference in terms of level of cognitive impairment between the residents and ICU physicians. Only cognitive flexibility appeared to be restored after 2 h of sleep. The other three cognitive skills were altered, regardless of the amount of sleep during the night shift. The cognitive abilities of intensivists were significantly altered following a night shift in the ICU, regardless of either the amount of professional experience or the duration of sleep during the shift. The consequences for patients' safety and physicians' health should be further evaluated.