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. It is an additional 30 minutes off with each successive day. Some days the patients have symptoms and other days there is no problem. Most patients attempt to sleep and arise at conventional social times. This produces progressively less sleep, with secondary daytime sleepiness interfering with functioning at work or at school.