title: Diurnal variability of total calcium during normal sleep and after an acute shift of sleep.
abstract: BACKGROUND
  Serum total calcium is becoming a widely used test when screening for hyperparathyroidism (HPT) and other causes of hypercalcemia, even if serum calcium is tightly regulated in the body it is unclear whether the reference values are correct for tests taken at different times of the day or for individuals with altered sleep patterns. Thus, the aim was to investigate how timing of testing and sleep affects serum calcium.
  
  
  METHODS
  The diurnal variation of total calcium in serum during night-time and day-time conditions was studied in seven healthy volunteers. Serum samples for calcium measurements were collected every hour (48 samples per individual) to evaluate the effect of sampling times, sleep and food intake on the test results.
  
  
  RESULTS
  The median intra-individual coefficients of variations for calcium were 3.3% during night-time sleep and 2.8% during day-time sleep conditions. There was a clear diurnal variation in serum calcium, with a trough at 08.00 h in the morning after night-time sleep and a difference of approximately 0.07 mmol/L between trough and peak. Calcium was lower around the end of the sleep periods, for both night-time and day-time sleep. Food intake did not affect calcium concentrations.
  
  
  CONCLUSIONS
  Evaluation of serum calcium results should take diurnal variation into account and allow slightly higher calcium values in the afternoon in comparison with samples collected in the morning.
authors.affiliation: Section of Clinical Chemistry, Department of Medical Sciences, Uppsala University, Sweden. peter.ridefelt@akademiska.se
authors.forename: Peter
authors.initials: P
authors.lastname: Ridefelt
authors.forename: John
authors.initials: J
authors.lastname: Axelsson
authors.forename: Anders
authors.initials: A
authors.lastname: Larsson
mesh_terms: D000328:Adult; D002118:Calcium / Q000097:blood*; D002940:Circadian Rhythm / Q000502:physiology*; D006801:Humans; D008297:Male; D012890:Sleep / Q000502:physiology*; D055815:Young Adult
chemical_list: D002118:Calcium