Non-24-hour sleep-wake rhythm disorder (N24SWD) is a type of circadian rhythm disorder, in which the individual's sleep patterns do not synchronize with the typical nighttime sleep periods. People with this condition struggle to maintain alignment between their sleeping and waking times.
People with N24SWD shift their sleep time later each day. As weeks pass, they find that they are increasingly out of sync with other people's wake up and sleep times.
Normal individuals possess a circadian rhythm that is more extended than 24 hours. Their sleep-wake clock gets reset to a 24-hour pattern every day by daylight and other habits. In the absence of light and this resetting, people’s sleep time will tend to continually linger later. Therefore, a large portion of the blind population are afflicted with N24SWD. Light serves as the key determinant for setting the brain's clock.
As your sleep pattern keeps advancing later in the day, it is easy to confuse with other circadian rhythm disorders. It may appear that you are suffering from delayed sleep-wake phase disorder if your bedtime gets continuously later until you are sleeping during the day. Then, it may look like advanced sleep-wake phase is at play when you start to fall asleep in early afternoon and evening. After more days, this cycle begins anew and your main sleep time does not remain fixed at the same hour every day, instead continuing to drift across the clock. Unless there are external disturbances, your sleep times remain uninterrupted with no breaks as seen with irregular sleep-wake rhythm disorder.