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Loss of light perception

Total blindness seldom happens all at once

Jennifer, age 60: "I lost my vision gradually over a course of 3 years. It didn't take long after I lost light perception for the cycle of Non-24 to start."

Total blindness is the lack of light perception. Onset can happen at different times. While some people are born blind and never perceive light, others are born sighted and lose their vision to illness or trauma.

Even some of the causes of blindness do not cause total blindness right away. It can take years, sometimes decades, to lose all light perception.

For example, those with congenital glaucoma and open-angle glaucoma may not reach total blindness until 60 years after onset. Retinitis pigmentosa can cause total blindness between 20 and 40 years later. And diabetic retinopathy can bring about total blindness 40 to 60 years after onset.

Total blindness and Non-24

When total blindness happens, chances are high that you will develop Non-24-Hour Sleep-Wake Disorder. As many as 70% of people who are totally blind suffer from the effects of living with Non-24. This serious circadian rhythm disorder brings about restlessness at night, the overwhelming urge to sleep during the day, and a range of functional problems at work or school. It's caused by the eye's inability to perceive environmental light, which is what resets the master body clock and circadian rhythms every day.

For most people, sighted or blind, the body clock is their natural timing mechanism. Everyone's body clock runs a little longer than 24 hours, and the extra time adds up. For example, if your body clock is 24.5 hours, today you're running a half hour behind. Tomorrow you're an hour behind, and so on until your natural rhythms have you sleeping during the day and awake at night. This continues and eventually your sleep-wake cycle briefly aligns with the typical day-night cycle. But just as quickly, it moves out of sync again. Some people experience a full circadian cycle in one and a half months. While, for others, it's several months before their sleep-wake cycle is realigned with the 24-hour day.

Light perception is a key element in maintaining a 24-hour body clock because environmental light signals the time of day to the brain. In people who are sighted, this resets the body clock to 24 hours, ensuring that the circadian rhythms synchronize to the typical day-night cycle. For people who are totally blind, there are no such light cues, and the body clock's extra minutes add up day by day until the circadian rhythms are essentially upside down from a typical 24-hour day.