Myopia Linked With Delayed Melatonin Circadian Timing

Young adults with myopia have significantly delayed melatonin circadian timing compared with their normally-sighted peers, according to a new study in the journal SLEEP.

Researchers in Australia recruited 18 participants with myopia and 14 without any refractive errors who were 18 years to 25 years old and examined them in a sleep laboratory, assessing their circadian timing using salivary dim light melatonin onset (DLMO). 

The study examined differences in timing of the melatonin circadian rhythm (DLMO timing), total melatonin output, and sleep outcomes between participant groups and investigated whether those differences were linked with the degree or severity of myopia.

Potential participants who had behavioral, medical or psychological variables that would interfere with circadian rhythms or interpreting eye measurements — including self-reported or diagnosed sleep disorders; use of medication that affected melatonin or dopamine levels; and a history of major eye disease or corrective refractive surgery — were excluded.

Participants were asked to keep a sleep diary for the 7 days before they arrived at the sleep laboratory, so researchers could keep track of their habitual sleep and daily activities. Sleep measures were taken from activity wristwatches.

During their night at the laboratory, scientists collected salivary DLMO from the participants each half hour for 7 hours, beginning 5 hours before, and ending 2 hours after, each patient went to sleep. The total melatonin produced was measured via aMT6s levels from urine voids collected from 6 PM and until wake-up time the following morning.

Patients from the myopia group experienced a DLMO-phase delay of more than an hour compared with the control. They also had a significant delay in falling asleep, shorter sleep duration, and more evening-type diurnal preferences.

The total amount of urine excreted overnight was not different between the 2  groups [t (30 df) = −0.558, P =.291]. However, the aMT6s levels derived from the total overnight volume of urine were significantly lower in the patients with myopia (29.17 ± 18.67) compared with participants without myopia (42.51 ± 23.97, Mann–Whitney rank sum test, P =.042). Additionally, the group with myopia demonstrated a phase-delay of 1 hour and 12 minutes in the salivary DLMO, compared with the  group without myopia [t (29 df) = 2.023, P =.026].

The research, through both objective and subjective measures, indicates poor overall sleep for those with myopia. They exhibited a significant delay in sleep onset time by 56 minutes compared with emmetropes [t (30 df) = −2.149, P =.019]. They also took longer to fall asleep after turning the lights out, spent less time in bed, and less time asleep than the group without myopia (all P <.05).  These findings are consistent with previous reports of poor sleep in patients with myopia.

The study also shows that the delay in DLMO is positively correlated with the degree of myopia. Patients with high myopia exhibited greater delay than those with less severe myopia.

Given their findings, the researchers suggest morning bright light therapy or early evening exogenous melatonin could be administered to phase-advance circadian timing and normalize sleep patterns in these patients.

Reference

Chakraborty R, Micic G, Thorley L, et al. Myopia, or near-sightedness, is associated with delayed melatonin circadian timing and lower melatonin output in young adult humans. SLEEP. Published online October 8, 2020. doi: 10.1093/sleep/zsaa208.