Fatigue has been implicated in the occurrence of critical incidents. Using a driving simulator to assess vigilance, we investigated the effect of sleep disruption on the ability of anaesthetists to maintain vigilance after a night on-call. Following an undisturbed night of sleep, subjects demonstrated a significant improvement in their vigilance scores. Subjects failed to demonstrate this following a disturbed night. The timing of disturbance had a bearing on performance. Subjects' performance was worse if their sleep had been disturbed in the first third of the night. The effect of sleep disruption on vigilance has implications for patient safety during anaesthesia.