title: Evidence of a unique and common genetic etiology between the CAR and the remaining part of the diurnal cycle: A study of 14 year-old twins.
abstract: BACKGROUND
  By and large, studies have reported moderate contributions of genetic factors to cortisol secreted in the early morning and even smaller estimates later in the day. In contrast, the cortisol awakening response (CAR) has shown much stronger heritability estimates, which prompted the hypothesis that the etiology of cortisol secretion may vary according to the time of day. A direct test of this possibility has, however, not yet been performed.
  
  
  OBJECTIVE
  To describe the specific and common etiology of the CAR, awakening level and cortisol change from morning to evening in an age-homogenous sample of twin adolescents.
  
  
  METHODS
  A total of 592 participants of the Québec Newborn Twin Study, a population-based 1995-1998 cohort of families with twins in Canada, have collected saliva at awakening, 30 min later, at the end of afternoon and in the evening over four collection days.
  
  
  RESULTS
  Multivariate Cholesky models showed both specific and common sources of variance between the CAR, awakening and cortisol diurnal change. The CAR had the strongest heritability estimates, which, for the most part, did not overlap with the other indicators. Conversely, similar magnitudes of genetic and environmental contributions were detected at awakening and for diurnal change, which partially overlapped.
  
  
  CONCLUSIONS
  Our study unraveled differences between the latent etiologies of the CAR and the rest of the diurnal cycle, which may contribute to identify regulatory genes and environments and detangle how these indicators each relate to physical and mental health.
authors.affiliation: School of Criminology, University of Montreal, Montreal, Canada; Research Center of the Montreal Mental Health University Institute, Montreal, Canada. Electronic address: isabelle.ouellet-morin@umontreal.ca.
authors.forename: Isabelle
authors.initials: I
authors.lastname: Ouellet-Morin
authors.affiliation: Department of Psychology, University of Quebec a Montreal, Montreal, Canada; Sainte-Justine Hospital Research Center, Montreal, Canada.
authors.forename: Mara
authors.initials: M
authors.lastname: Brendgen
authors.affiliation: Sainte-Justine Hospital Research Center, Montreal, Canada.
authors.forename: Alain
authors.initials: A
authors.lastname: Girard
authors.affiliation: Research Center of the Montreal Mental Health University Institute, Montreal, Canada; Department Psychiatry, University of Montreal, Montreal, Canada.
authors.forename: Sonia J
authors.initials: SJ
authors.lastname: Lupien
authors.affiliation: Department of Psychology, Laval University, Quebec, Canada.
authors.forename: Ginette
authors.initials: G
authors.lastname: Dionne
authors.affiliation: Sainte-Justine Hospital Research Center, Montreal, Canada; School of Psychoéducation, Université de Montreal, Montreal, Canada.
authors.forename: Frank
authors.initials: F
authors.lastname: Vitaro
authors.affiliation: Department of Psychology, Laval University, Quebec, Canada; Institute of Genetic, Neurobiological, and Social Foundations of Child Development, Tomsk State University, Russian Federation.
authors.forename: Michel
authors.initials: M
authors.lastname: Boivin
mesh_terms: D000293:Adolescent; D000328:Adult; D002940:Circadian Rhythm / Q000235:genetics* / Q000502:physiology; D015331:Cohort Studies; D005260:Female; D006801:Humans; D006854:Hydrocortisone / Q000378:metabolism*; D007030:Hypothalamo-Hypophyseal System / Q000378:metabolism; D008297:Male; D010913:Pituitary-Adrenal System / Q000378:metabolism; D011792:Quebec; D012463:Saliva / Q000378:metabolism; D013312:Stress, Physiological / Q000235:genetics; D013997:Time Factors; D014427:Twins / Q000235:genetics; D014851:Wakefulness / Q000235:genetics*
chemical_list: D006854:Hydrocortisone
keywords: Cortisol; Cortisol awakening response; Diurnal rhythm; Genes; HPA axis; Twin studies