Some eye diseases can trigger serious sleep disorders, a study by doctors in the United States suggests.
The doctors believe optic nerve damage disturbs sleep patterns
They say people with damage to their optic nerve, which connects the eye to the brain, can have problems sleeping.
They can have difficulty falling asleep and can wake up at strange times. They can also be sleepy during the day and suffer from insomnia at night.
Writing in the journal Ophthalmology, they say the findings highlight the need to treat these patients early.
Dr Russell van Gelder and colleagues at Washington University Medical School in St Louis studied 25 visually impaired young people between the ages of 12 and 20.
Half had optic nerve damage while the remainder had other eyesight problems.
The researchers compared these volunteers with 12 healthy young people.
All of those involved in the study wore a watch-like device for two weeks, which enabled the researchers to monitor their circadian rhythms or body clocks.
"The study showed the subjects with optic nerve disease were 20 times more likely to have pathologic levels of daytime sleepiness, as indicated by napping, than the subjects with normal sight," said Dr van Gelder.
"They were also nine times more likely to have pathologic sleepiness than the visually impaired subjects who were blind from the non-optic nerve diseases.
"We suspect these patients have difficulty using daylight to synchronize their internal rhythms to the outside world."
Dr van Gelder said the findings were surprising.
"Taken together, these results lead to the unexpected conclusions that eye disease is a risk factor for sleep disorders and whether the optic nerve is healthy or diseased strongly influences the risk of sleep disorders."
Dr Alfredo Sadun, professor of ophthalmology at the Keck-University of Southern California, said the findings should prompt doctors to ensure patients with optic nerve damage were treated as quickly as possible.