The body clock is your natural timing mechanism, and for most people, sighted or blind, it runs a little longer than 24 hours. Thus, it's a non-24-hour clock. For some, it runs just a few minutes longer, and for others it runs much longer. The reason for this is not known.
For example, if your body clock is 24.5 hours, today you're running a half hour behind. Tomorrow you're an hour behind, and so on, until your natural rhythms have you sleeping during the day and awake at night. This continues and eventually your sleep-wake cycle briefly syncs up with the typical day-night cycle. Then it begins to move out of sync again. Some people experience a full circadian cycle as short as one and a half months. For others, it can be several months before their sleep-wake cycle is realigned with the 24-hour day.
The eye has two functions: to allow us to see images and to take in light. This light then signals the time of day to the brain. In people who are sighted, the non-24-hour master body clock is reset every day to 24 hours in the same way that hands on a clock can be reset. This ensures that the circadian rhythms synchronize to the typical day-night cycle.
For people who are totally blind, there are no such light cues. The body clock is left to run its natural course, with extra minutes adding up day by day until your circadian rhythms are essentially upside down from a typical 24-hour day.
For more information and support on this condition go to the official Non-24 web site at http://www.non-24.com/