title: Nocturnal elevation of intraocular pressure is detectable in the sitting position.
abstract: OBJECTIVE
  When intraocular pressure (IOP) was monitored in supine healthy young adults throughout a 24-hour period, a diurnal-to-nocturnal elevation of IOP was observed. This study was undertaken to investigate whether a similar elevation of IOP can be detected when experimental subjects are in the sitting position.
  
  
  METHODS
  Experimental subjects were 16 nonsmoking, healthy young volunteers (ages, 18-25 years). Subjects with myopia of more than 3 D were excluded. They were housed in a sleep laboratory for 24 hours in a strictly controlled environment. An 8-hour nocturnal/sleep period was assigned to each volunteer according to the individual's accustomed sleep cycle. IOP was measured every 2 hours with a pneumatonometer with the volunteers in both sitting and supine positions. Mean diurnal-to-nocturnal IOP change and the cosine-fit 24-hour IOP rhythm were compared between the sitting and the supine IOP data.
  
  
  RESULTS
  Mean IOP was significantly higher in the nocturnal period than in the diurnal/wake period for both the sitting and the supine IOPs. The 24-hour IOP troughs appeared at the end of the diurnal period, and the peaks appeared at the end of the nocturnal period. The difference between the trough and the peak was 3.8 +/- 0.6 mm Hg (mean +/- SEM) in the sitting position and 3.4 +/- 0.6 mm Hg in the supine position. Cosine-fitting of 24-hour IOP data showed a synchronized 24-hour rhythm of the sitting and the supine IOPs for the group. There was no difference in the phase timing or the magnitude of variation between these two 24-hour rhythms of sitting and supine IOPs.
  
  
  CONCLUSIONS
  A nocturnal elevation of IOP can be detected in healthy young adults in both the sitting and the supine positions. There is a 24-hour rhythm of sitting IOP that is not different from the 24-hour rhythm of supine IOP.
mesh_terms: D000293:Adolescent; D000328:Adult; D002940:Circadian Rhythm / Q000502:physiology*; D005260:Female; D006801:Humans; D007429:Intraocular Pressure / Q000502:physiology*; D008297:Male; D011187:Posture / Q000502:physiology*; D012890:Sleep / Q000502:physiology; D016683:Supine Position; D014065:Tonometry, Ocular