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Every one% increase in blue light lens transmission reduced the odds of sleep disturbance by five%. The association was similar across age groups and remained significant after controlling for a variety of potential confounders, as reported in the September issue of Sleep.
"To translate the odds ratios into a clinical example, a 50-year-old, nonsmoking, nondiabetic female with a low risk of ischemic heart disease who was in the 97.5% upper normal range of blue light lens transmission ... had a risk of sleep disturbances of 16.4%," Line Kessel, MD, PhD, of Glostrup Hospital and the University of Copenhagen, and co-authors wrote.
"[In contrast] a similar ... 50-year-old female who was in the 2.5% lower normal range of blue light lens transmission ... had a risk of having sleep disturbances of 37.9%."
The odds of sleep disturbance also increased significantly with the extent of autofluorescence, a measure of lens transmission and yellowing.
Regulation of sleep patterns and circadian rhythms occurs through the retinohypothalamic tract in response to stimulation of retinal ganglions, primarily by blue light.
With aging, the lens acquires a yellowish discoloration from accumulation of chromophores that preferentially absorb in the short wavelength region of the light spectrum. The authors hypothesized that lens yellowing might act as a filter for blue light in older people, who are more prone to sleep disturbance.
"Theoretically, the aging process of the lens of the eye may be an important causative factor in sleep disorders," the authors wrote.
To examine the relationship between lens aging and sleep disturbance, investigators recruited participants from an ongoing epidemiologic study (Inter99 Eye Study) of associations between ophthalmic parameters and health.
The study of lens aging and sleep disturbance involved 970 people, ages 30 to 60. The study population consisted of 142 participants with a high risk of ischemic heart disease, 107 with newly diagnosed type 2 diabetes, 11 participants with a history of diabetes, 147 participants with impaired glucose tolerance, and a miscellaneous group of 61 participants. The control group had 502 participants.
Investigators assessed sleep disturbance by means of questionnaires and prescriptions for sleeping medications. Lens aging was determined by autofluorometry. Sleep disturbance was defined as answering "yes" to "having trouble sleeping and/or purchasing a sleep medication in the past year."
Overall, the prevalence of sleep disturbance was 24.4%. and more than 80% of participants with sleep disturbances said the problem occurred frequently and required treatment with sleep medication. The prevalence ranged from a high of 32.5% of participants, ages 55 to 60, to a low of 15.7% in participants ages 30 to 35 (P=0.0002).
Sleep disturbance affected women more often than men (32.2% versus 18.4%, P<0.0001). It also was more common in participants with diabetes (30.7% versus 22.5% for normoglycemic individuals, P=0.016) and smokers (28.6% versus 21.0% of nonsmokers, P=0.007).
The inverse association between sleep disturbance and blue light lens transmission was represented by an odds ratio (OR) of 0.95 per 1% increase in lens transmission (95% CI 0.93 to 0.97, P<0.0001).
Adjustment for age, sex, diabetes, smoking, and risk of ischemic heart disease resulted in an OR of 0.97 (95% CI 0.95 to 0.99, P=0.016).
The findings are consistent with the hypothesis that decreased blue light transmission reduces stimulation of retinal ganglions containing melanopsin, which has a key role in photoentrainment of circadian rhythms.
"Melanopsin is stimulated by predominantly blue light with a maximum absorption around 480 nm in humans, the very part of the visible spectrum that is mostly affected by the aging process of the human lens," the authors wrote.
"The results of the present study indicate that the spectral characteristics of the light reaching the retina, and specifically the total amount of blue light that penetrates the refractive media of the eye, may have a profound impact on sleep quality, most likely mediated via reduced ocular photic regulation of melatonin secretion," they added.
"Our results support that choosing the right indoor lighting conditions may have a beneficial effect on sleep and that blue light therapy might be used to modulate circadian sleep disorders," they concluded.
The study had some limitations including the risk that some patients were misclassified because sleeping medication may be used for other conditions, such as benzodiazepines for anxiety.