Non-24-hour sleep-wake disorder (Non-24) is a circadian rhythm disorder and happens when your internal clock is not synched with light and dark cycles of the day and night. Most people who have it are totally blind. That's because your internal clock gets its cue from the light you see. But sometimes people who have normal vision get it.
When you have non-24, you may find yourself gradually going to bed later every night and waking up later each day.