London, Sep 1 (IANS): Yellowing of the eye lens that absorbs blue light may be causing sleep disorders among the elderly, says a study.

Blue light is a portion of the visible-light spectrum that influences the normal sleep cycle by triggering release of melatonin in the brain. Melatonin helps signal when it is time to be sleepy or alert.

"The strong link between lens yellowing and age could help explain why sleep disorders become more frequent with increasing age," said Line Kessel, scientist and ophthalmologist at Glostrup Hospital, Denmark, who led the study.

About 970 volunteers in Denmark had their eyes examined for determining how much blue light is transmitted into the retina, the journal Sleep reported.

Volunteers were considered to have a sleep disorder if they confirmed that they "often suffer from insomnia" or if they purchased prescription sleeping pills within the last 12 months, according to a Glostrup statement.

Of those classified as having a sleep disturbance, 82.8 percent affirmed that they both suffered from insomnia and used sleep medication.

"The results showed that while age-related lens yellowing is of relatively little importance for visual function, it may be responsible for insomnia in the elderly," said Kessel.

Significantly higher rates of sleep disorders were reported by older participants, women, smokers and those with diabetes mellitus. Previous studies have shown that diabetes and smoking accelerate the rate of lens aging.

Kessel said another important factor to consider was that sleep quality has been shown to improve after cataract surgery. "The transmission of blue light currently cannot be improved by any other method than cataract surgery."