Non-24 simply means a person’s circadian rhythm is not on a 24-hour day. Most Non-24 patients are blind, but sighted people can have it, too.
In totally blind people, the lack of light perception throws the circadian rhythm out of sync; their circadian rhythms are typically normal otherwise. This may make it easier for blind patients to entrain to a normal 24 hour day using medication.
In sighted people, there is no singular cause and many factors can be at play. Sighted Non-24 is typically more complex and may not respond to the same therapies as cases of Non-24 caused by blindness.