In most people, the master body clock runs slightly longer than 24 hours. What this means is that rather than cycle on a 24-hour day, most people's natural rhythms actually cycle a bit longer. Whether the cycle runs two minutes or 30 minutes longer, if you have Non-24 these minutes add up day after day, a few one day adding to a few more the next, eventually causing a noticeable change in the times during the day when your body expects to sleep and expects to be awake.
Though Non-24 may appear to be a sleep disorder, it isn't. It's actually a serious, chronic circadian rhythm disorder very common in people who are totally blind, and it can arise at any age. Currently, there are 1.3 million people who are legally blind in the United States. Of the legally blind, 130,000 have no light perception (i.e., totally blind), and as many as 70% suffer from Non-24.