title: Diurnal axial length fluctuations in human eyes.
abstract: OBJECTIVE
  This study sought diurnal variations of eye length in human subjects, analogous to those reported in laboratory animals.
  
  
  METHODS
  Seventeen subjects, ages 7 to 53 (median 16) years and mean spherical equivalent refractive error -0.68 D (range, -3.00 to +1.00 D), underwent axial length measurements at multiple times during the day between 7 AM and 1 AM the following day, using partial coherence interferometry (PCI), a highly precise, noncontact method. Diurnal axial length measurements were obtained on two or more days in 10 of these subjects.
  
  
  RESULTS
  During at least 1 day, 15 subjects showed a statistically significant (ANOVA, P < 0.05) diurnal fluctuation of axial length, with a magnitude generally between 15 and 40 microm. From the diurnal tracings that fit a sine curve using statistical criteria, the mean period of fluctuation was 21.6 +/- 4.33 hours (SD), the mean amplitude was 27.1 +/- 11.9 microm (SD; range, 12.8-41.4 microm), and the maximum axial length tended to occur at midday. Each of the subjects with multiple daily measurements showed axial length fluctuations on at least 1 day, but there were day-to-day differences in the diurnal variations: most notably, four subjects showed axial length fluctuations on each day; in others, the fluctuations were not observed on each testing day.
  
  
  CONCLUSIONS
  The human eye undergoes diurnal fluctuations in axial length, with a pattern suggesting maximum axial length at midday. Based on repeated measurements, these daily fluctuations may not appear regularly in all subjects, suggesting the possibility of physiologic influences that must be defined.
authors.affiliation: Division of Pediatric Ophthalmology, The Children's Hospital of Philadelphia, Philadelphia, Pennsylvania 19104, USA.
authors.forename: Richard A
authors.initials: RA
authors.lastname: Stone
authors.forename: Graham E
authors.initials: GE
authors.lastname: Quinn
authors.forename: Ellie L
authors.initials: EL
authors.lastname: Francis
authors.forename: Gui-shuang
authors.initials: GS
authors.lastname: Ying
authors.forename: D Ian
authors.initials: DI
authors.lastname: Flitcroft
authors.forename: Parag
authors.initials: P
authors.lastname: Parekh
authors.forename: Jamin
authors.initials: J
authors.lastname: Brown
authors.forename: Joshua
authors.initials: J
authors.lastname: Orlow
authors.forename: Gregor
authors.initials: G
authors.lastname: Schmid
mesh_terms: D000293:Adolescent; D000328:Adult; D002648:Child; D002940:Circadian Rhythm / Q000502:physiology*; D003941:Diagnostic Techniques, Ophthalmological; D005123:Eye / Q000033:anatomy & histology*; D005260:Female; D006801:Humans; D007368:Interferometry / Q000379:methods; D008027:Light; D008297:Male; D008875:Middle Aged; D009799:Ocular Physiological Phenomena