Heart beats, body temperature, breathing patterns, and blink rates are examples of ultradian rhythms. A woman's menstrual cycle is infradian. All of these cycles are governed by hormones released by internal body clocks. Our seven day weeks, our calendar year, our day divided into 24 hours, all may seem like inventions of modern society. But they are not.
Nature imposes rhythms that are weekly, monthly, and daily on all forms of animal and insect life.
We fashioned our time pieces on the model that nature provided for us. Anything that disrupts these systems has a far reaching impact on the entire human body. Because they lack photic input to the circadian time keeping system, people who are totally blind have body rhythms that are not necessarily synchronized to the 24 hour solar and social day. Insomnia is the most common sleep complaint of people who are blind. A free running clock goes by a 24 hour, plus thirty minute cycle daily.