Some bodily activities, sleeping, for instance, mostly occur once every 24 hours; they follow a circadian rhythm. Other bodily functions, such as body temperature, cognitive performance and blood pressure, present an additional 12-hour cycle, but little is known about the biological basis of their rhythm. A team of scientists from various institutions, including Baylor College of Medicine, has revealed that, in addition to 24-hour clocks, mammals and other organisms have 12-hour clocks that are autonomous,

Source: A 12-hour biological clock coordinates essential bodily functions