**4.4 Non-24 sleep-wake rhythm disorder (N24SWRD)**

The etiology of N24SWD is related to disruption of the portion of the circadian system responsible for capturing photic stimuli in the retina. Pathologies that lead to vision loss often but not necessarily impact this system. Thus, N24SWD occurs most often in blind individuals, with reports of 50% of blind patients diagnosed with N24SWD and up to 70% of blind individuals having symptoms of chronic sleep disturbance [2, 3, 7]. Men are disproportionally affected compared to women at a ratio of >2:1. N24SWD is rare, but has been reported, in sighted individuals. In sighted individuals, N24SWD is thought to be a severe form of delayed sleep phase disorder where entrainment to light can no longer be effective [7].
