title: Development of Salivary Cortisol Circadian Rhythm and Reference Intervals in Full-Term Infants.
abstract: BACKGROUND
  Cortisol concentrations in plasma display a circadian rhythm in adults and children older than one year. Earlier studies report divergent results regarding when cortisol circadian rhythm is established. The present study aims to investigate at what age infants develop a circadian rhythm, as well as the possible influences of behavioral regularity and daily life trauma on when the rhythm is established. Furthermore, we determine age-related reference intervals for cortisol concentrations in saliva during the first year of life.
  
  
  METHODS
  130 healthy full-term infants were included in a prospective, longitudinal study with saliva sampling on two consecutive days, in the morning (07:30-09:30), noon (10:00-12:00) and evening (19:30-21:30), each month from birth until the infant was twelve months old. Information about development of behavioral regularity and potential exposure to trauma was obtained from the parents through the Baby Behavior Questionnaire and the Life Incidence of Traumatic Events checklist.
  
  
  RESULTS
  A significant group-level circadian rhythm of salivary cortisol secretion was established at one month, and remained throughout the first year of life, although there was considerable individual variability. No correlation was found between development of cortisol circadian rhythm and the results from either the Baby Behavior Questionnaire or the Life Incidence of Traumatic Events checklist. The study presents salivary cortisol reference intervals for infants during the first twelve months of life.
  
  
  CONCLUSIONS
  Cortisol circadian rhythm in infants is already established by one month of age, earlier than previous studies have shown. The current study also provides first year age-related reference intervals for salivary cortisol levels in healthy, full-term infants.
authors.affiliation: Department of Clinical and Experimental Medicine, Division of Pediatrics, Faculty of Health Sciences, Linköping University, Linköping, Sweden.
authors.forename: Katrin
authors.initials: K
authors.lastname: Ivars
authors.affiliation: Department of Clinical and Experimental Medicine, Division of Pediatrics, Faculty of Health Sciences, Linköping University, Linköping, Sweden; Department of Pediatrics, Institute of Clinical Sciences, Gothenburg University, Gothenburg, Sweden.
authors.forename: Nina
authors.initials: N
authors.lastname: Nelson
authors.affiliation: Division of Neuroscience, Department of Clinical and Experimental Medicine, Faculty of Health Sciences, Linköping University, Department of Neurosurgery, Anaesthetics, Operations and Specialty Surgery Center, Region Östergötland, Linköping, Sweden.
authors.forename: Annette
authors.initials: A
authors.lastname: Theodorsson
authors.affiliation: Division of Microbiology and Molecular Medicine, Department of Clinical and Experimental Medicine, Faculty of Health Sciences, Linköping University, Department of Clinical Chemistry, Center for Diagnostics, Region Östergötland, Linköping, Sweden.
authors.forename: Elvar
authors.initials: E
authors.lastname: Theodorsson
authors.affiliation: Division of Microbiology and Molecular Medicine, Department of Clinical and Experimental Medicine, Faculty of Health Sciences, Linköping University, Department of Clinical Chemistry, Center for Diagnostics, Region Östergötland, Linköping, Sweden.
authors.forename: Jakob O
authors.initials: JO
authors.lastname: Ström
authors.affiliation: Department of Social and Welfare Studies, Division of Health, Activity and Care, Faculty of Health Sciences, Linköping University, Linköping, Sweden.
authors.forename: Evalotte
authors.initials: E
authors.lastname: Mörelius
mesh_terms: D000328:Adult; D002657:Child Development; D002940:Circadian Rhythm; D005260:Female; D006801:Humans; D006854:Hydrocortisone / Q000032:analysis / Q000378:metabolism*; D007223:Infant; D008016:Life Change Events; D008137:Longitudinal Studies; D008297:Male; D011446:Prospective Studies; D012463:Saliva / Q000737:chemistry / Q000378:metabolism*
chemical_list: D006854:Hydrocortisone