Reduced sleep is also related to an increase in both cardiovascular events and cardiovascular disease. In the Nurses' Health Study, researchers found evidence of increased risk of coronary events in female subjects who obtained seven hours or less of sleep per night compared to those who averaged eight hours per night. In another study, a two- or three-fold increase in risk of cardiovascular events, such as stroke or myocardial infarction, was found in subjects with an average sleep duration of five hours or less per night (or chronically having five hours or less of sleep per night at least twice per week) was reported. Similar findings have been observed in studies examining cardiovascular health in shift workers, who typically experience disruptions in their circadian rhythm. Although certain of a link between short sleep and cardiovascular risk, researchers do not understand the reason for this.