9. Non-24-hour Sleep-wake Syndrome
This disorder causes disruption in the normal 24-hour sleep cycle that govern when we are awake and when we feel the need for sleep. People who suffer from this disorder often endure periods of time where they are not able to sleep in accordance with their normal schedule. In all but very rare cases, this disorder affects the blind, which is probably another indicator that light is key to helping us “set” and “adjust” our internal clock so that we are able to maintain a consistent sleep cycle.