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Rigmor C Baraas, Nickolai Godtfred Nilsen, Hilde Rogeberg Pedersen, Lene A Hagen, Stuart J. Gilson, Kenneth Knoblauch; Seasonal and diurnal variation in saliva melatonin and ocular axial length at 60° latitude North.. Invest. Ophthalmol. Vis. Sci. 2020;61(7):1923.
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Late circadian timing is reported to be associated with negative general health outcomes in humans and myopic eye growth in animals. Whether late circadian timing also causes myopic eye growth in humans remains unknown. Here we investigated diurnal changes in ocular axial length and melatonin (Mel) between winter (6 hours of daylight/day) and summer (18 hours of daylight/day) in young healthy adults.
Twenty healthy students aged 17–24yrs [10 females; 5 myopes (≤-0.50D), 6 emmetropes, 9 hyperopes (≥0.50D) as measured with cycloplegic autorefraction (Huvitz HRK-8000A)] were included in the study. Saliva samples and measurements of ocular axial length (AL: IOLMaster 700) were taken at 1 and 4 hours post habitual wake-up time (HWT), in the afternoon (AF), and every hour from 4 hours prior to habitual sleep time (HST) until 1 hour post HST. HWT and HST were confirmed wearing Actigraph GT3X for a week prior to measurements. Participants refrained from beverage/food consumption known to affect Mel levels. Participants remained in the lab with light levels below 20 lux from HST-4 and did not engage in any near work. They viewed a movie at 5 m for 15 minutes prior to each set of measurements to relax their accommodation. The saliva Mel levels [pg/mL] and AL [mm] profiles were modelled using R software by estimating the mesor, amplitude and acrophase of the best fitting 24-hour period sine-wave.
There was a shift in acrophase from winter to summer, with about an hour earlier circadian timing in summer for both Mel levels (62 min, p<0.0001) and AL (51 min, p=0.06). Longer AL was associated with later circadian timing in winter and a strong correlation was observed between AL mesor and the size of the AL acrophaseshift; the longer the eye the larger was the phaseshift (r2=0.99, p<0.0001). AL growth > 0.06 mm was observed in 3 myopes, 1 hyperope and 2 emmetropes, who all had winter AL >24.1 mm except for one, who also became myopic between winter and summer.
Late circadian timing in winter and larger shifts in summer were associated with longer ocular axial length and accelerated eye growth. This agrees with what has been reported for animals (Nickla, Exp. Eye Res. 2013, 114, 25–34), implying that bodily diurnal rhythms are closely associated with ocular diurnal rhythms and eye growth regulation in humans too.
This is a 2020 ARVO Annual Meeting abstract.
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