Why you should avoid heart surgery on a Friday: It's the most dangerous times to year to be treated Study links date and time of surgery with risk of death Patients admitted to hospital in January, on a Friday or overnight had a greater risk of death from heart failure Meanwhile, patients treated on a Monday had the shortest hospital stays and the lowest mortality rates Data showed that admissions for heart failure peaked in February, but the majority of deaths recorded in January The precise month, day and even time patients undergo heart surgery has been linked to a greater risk of dying during treatment. In a study of nearly one million hospital admissions for heart failure over 14 years, researchers pinpointed the times associated with the highest risk of death and longest stays in the hospital. The data, presented at the Heart Failure Congress 2013 in Lisbon, was collated from admissions to New York hospitals between 1994 and 2007. As well as the number of admissions increasing, death rates and length of stays declined - which researchers said was due to an improved standard of treatment. But most notably, the study found that patients admitted to the hospital in January, on a Friday or overnight, ended up spending more time in the hospital and had a greater risk of death from heart failure than those admitted at other times. While daily admissions for heart failure peaked in February, the majority of deaths and longest admissions were recorded in January. Meanwhile, patients treated on a Monday had the shortest hospital stays and the lowest mortality rates. Various theories have been suggested to explain the cause of seasonal variations in heart failure morbidity and mortality, for example that the holiday spike is caused by alcohol and drug use. The new data may help hospitals and doctors which times of year and day may need more vigilance However, Dr David Kao, a fellow of cardiovascular medicine at the University of Colorado School of Medicine, who led the study, said: 'For the first time, we've shown that there wasn't a higher rate of alcohol and drug use reported in heart failure patients during December and January, when heart failure mortality was the highest.' An increase in pneumonia in the winter could also impact on heart failure mortality, but there was less seasonal variation in other respiratory diseases like chronic obstructive pulmonary disease (COPD). It's thought that the data will advise hospitals and doctors which times of year and day may need more vigilance. Experts also recommend that patients with heart issues are aware of higher risk times for heart events. 'People often avoid coming into the hospital during the holidays because of family pressures and a personal desire to stay at home, but they may be putting themselves in danger,' Dr Kao. The findings suggest that staffing may have an impact on seasonal variations in mortality and length of stay.