A considerable amount of attention has been paid to the issue of physician work hours and patient safety, thanks in part to regulations limiting duty hours for resident physicians. Fatigue has also been demonstrated to be a risk factor for errors among nurses, particularly when nurses work overtime or extended duration shifts. In this analysis, the authors found that among nurses, working voluntary overtime or working more than 40 hours per week were strongly correlated with an increase in self-reported errors, particularly needlestick injuries and medication errors. While legislative efforts have focused on restricting mandatory overtime for nurses, these results raise concern that the widespread practice of taking voluntary overtime shifts could negatively affect patient safety.