More importantly, it is safe to go in for bypass surgery any day of the week or any time of the day, according to the study reported in the journal Anesthesiology by Dr. Daniel I. Sessler and his colleagues. That may not be such an obvious conclusion. Cars made in Detroit on Monday mornings have, at least in the past, been notorious for containing above-normal levels of defects because of hangovers among workers. The Chernobyl nuclear reactor disaster and the Exxon Valdez grounding both occurred during the 2 a.m. to 5 a.m. period when circadian rhythms are at their lowest. Fatigue from long hours and working nights contributes to as much as 20 percent of all transportation accidents. It is not unreasonable, therefore, to ask whether fatigue can have a similar effect on the medical profession. One previous study, for example, found that the probability of anesthesia-related adverse events reportedly increases from a low of 1 percent at 9 a.m. to 4.2 percent by 4 p.m., when personnel are more tired. Because of its well-established protocols and outcome measures, coronary artery bypass surgery -- commonly known as CABG -- provides a good way to examine the problem, the authors wrote. They looked at records of 18,597 patients who underwent elective coronary artery bypass surgery at the Cleveland Clinic between Jan. 1, 1993, and July 1, 2006. They found that the success rate was constant, regardless of the time of day, the day of the week, the month of the year and, of course, the phase of the moon. Granted, elective coronary artery bypass surgery is rarely performed between 5 p.m. and 5 a.m., when fatigue would be expected to play a bigger role in determining outcomes. But at least for elective surgeries, their results suggest that you can be confident no matter when your procedure is scheduled. 