title: Effect of day length on eye growth, myopia progression, and change of corneal power in myopic children.
abstract: OBJECTIVE
  Because of the northern location of Denmark, the length of the day over the year varies from 7 to 17.5 hours. Experimental and clinical results suggest that the development of myopia may be related to ambient light exposure. The purpose of current study was to investigate whether axial eye growth, myopia progression, or corneal power change in Danish myopic children varies with the length of the day.
  
  
  METHODS
  Cross-sectional study.
  
  
  METHODS
  Two hundred thirty-five children 8 to 14 years of age found to have myopia during screening for a clinical trial (ClinicalTrial.gov identifier, NCT00263471; accessed December 6, 2005). All children found to have any value of spherical equivalent that was myopic (<0 diopters [D]) at the first of 2 visits were included.
  
  
  METHODS
  Cycloplegic refraction was measured using an autorefractor, axial eye length, and corneal power using an automatic combined noncontact partial coherence interferometer and keratometer. The accumulated number of daylight hours during the measurement period was calculated for each participant using an astronomical table.
  
  
  METHODS
  Change over 6 months in axial length, refraction, and corneal power.
  
  
  RESULTS
  Accumulated hours of daylight ranged from 1660 to 2804 hours. Significant correlations were found between hours of daylight and eye elongation (P = 0.00), myopia progression (P = 0.01), and corneal power change (P = 0.00). In children with an average of 2782 ± 19 hours of daylight, axial eye growth was 0.12 ± 0.09 mm, myopia progression was 0.26 ± 0.27 D, and corneal power change was 0.05 ± 0.10 D per 6 months, whereas in children with an average of 1681 ± 24 hours of daylight, axial eye growth was 0.19 ± 0.10 mm, myopia progression was 0.32 ± 0.27 D, and corneal power change was -0.04 ± 0.08 D per 6 months.
  
  
  CONCLUSIONS
  Eye elongation and myopia progression seem to decrease in periods with longer days and to increase in periods with shorter days. Children should be encouraged to spend more time outside during daytime to prevent myopia.
  
  
  BACKGROUND
  The author(s) have no proprietary or commercial interest in any materials discussed in this article.
authors: State Key Laboratory of Ophthalmology, Zhongshan Ophthalmic Center, SunYat-sen University, Guangzhou, People's Republic of China. sarah72@126.com
authors: Dongmei
authors: D
authors: Cui
authors: Klaus
authors: K
authors: Trier
authors: Søren
authors: S
authors: Munk Ribel-Madsen
mesh_terms: D000293:Adolescent; D058452:Axial Length, Eye / Q000503:physiopathology; D002648:Child; D003315:Cornea / Q000503:physiopathology*; D003718:Denmark; D018450:Disease Progression; D005123:Eye / Q000254:growth & development*; D005260:Female; D006801:Humans; D007368:Interferometry; D008297:Male; D009216:Myopia / Q000473:pathology / Q000503:physiopathology*; D017440:Photoperiod; D012029:Refraction, Ocular / Q000502:physiology