Non-24-hour sleep-wake disorder (non-24 disorder), also known as hypernychthemeral syndrome, is a circadian rhythm sleep disorder characterized by non-synchronization to a 24-hour day leading to insomnia and daytime sleepiness with sometimes severe associated manifestations.

Blindness is the condition of lacking visual perception defined as visual perception below 3/60 and/or a visual field of no greater than 10 degress in radius around central fixation.