A new study has shed some light on a serious and little-known sleep condition called 'Non-24-Hour Sleep-Wake Disorder', and why it seems to primarily affect totally blind people.

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According to the Toronto's The Star, preliminary results of an ongoing clinical trial found an estimated 65,000 to 95,000 blind people in the United States who have sleep complaints, up to 70% might suffer from non-24:
The human body clock consists of an intricate network of chemical and electrical signals controlled by two rice-grain-size structures deep in the brain. Most people’s internal clock runs slightly longer than 24 hours. However, among sighted people, the clock is reset each day by light-sensing cells in the eyes that signal to the brain that it is daytime.

For the blind, this reset mechanism fails. The resulting symptoms are similar to those experienced by sighted people who chronically disrupt their light cycle by shift work or travel across time zones.
Despite not being able to sense light, the condition is especially prevalent among blind people. Below is a little more explanation on how devastating this condition can be to your daily life:
In theory, a blind person with an internal body clock of 24.5 hours may feel ready to fall asleep at 10:30 p.m. on Monday but not be able to fall asleep until 11 p.m. on Tuesday. This cycle is unrelenting, making those affected want to fall asleep later and later each day.
Currently, there are no FDA-approved medications to treat non-24. Some sufferers have found limited relief through treatment with synthetic versions of the sleep-inducing hormone melatonin, which can reset the body clock by providing a chemical pulse to the brain that signals nighttime. A candidate drug called tasimelteon, which has a similar molecular structure to melatonin, is also being developed by Washington, D.C.,-based Vanda Pharmaceuticals.