Long sleep duration was significantly associated with the risk of DR, and short sleep duration was significantly associated with the risk of cataract. Photo: Bruce Mars on Unsplash.

Inappropriate sleep duration has already been defined as a modifiable risk factor for several health outcomes at the population level, including diabetes, metabolic syndrome and all-cause mortality, but the associations between length of sleep and major eye disorders remain controversial. A recent literature review published in Eye compiled available evidence and revealed that long sleep duration coincided with increased risk of diabetic retinopathy (DR), while short sleep duration was correlated with significantly elevated odds for cataract. Notably, long sleep duration per day, including nighttime sleep and daytime napping, rather than per night was significantly associated with an increased risk of DR.

Still, the researchers prefaced that their analysis was performed mainly based on cross-sectional data, which is unable to infer the direction of the causality between two variables. Also, the majority of relevant literature assessed sleep length merely on the basis of self-report, which may be subject to recall bias.

For articles reporting multiple categories of sleep durations, long and short sleep were defined as the longest and shortest categories reported in the original article, respectively. In those studies where both subjective (e.g., self-reported questionnaires) and objective (via actigraphy or polysomnography) sleep duration were reported, only the latter was applied in this paper.

The researchers identified 21 relevant articles comprising data on 777,348 participants; of these, 17 were cross-sectional studies, three were longitudinal and one was case-controlled. The pooled results indicated that long sleep duration was significantly associated with the risk of DR (OR: 1.84) and short sleep duration was significantly associated with the risk of cataract (OR: 1.20). A significant relationship was observed between the risk of DR and long sleep duration per day (i.e., nighttime sleep plus daytime napping, OR: 1.74) rather than per night (OR: 2.17). The length of long sleep duration (i.e., >10 hours per night) increased the risk of myopia (OR: 1.02).

“The evidence for a link between sleep duration and major eye disorders is accumulating, whereas the available literature has not yet been systematically reviewed,” the researchers wrote in their paper. “These findings could contribute to the growing knowledge on the possible relationship between circadian rhythms and eye disorders.”

In addition to the caveats about direction of causality and reliability of self-reported data, the authors cautioned that the sleep duration category within each study and the criteria for defining major eye disorders varied across studies, which restricts the power to judge the impact of sleep duration on major eye disorders.

“Future studies are needed to shed light on the causal directions and biological mechanisms behind the relationships between sleep behaviors and major eye disorders,” they concluded. “Whether interventions to normalize sleep duration could improve vision health remains an open question.”