title: The effect of the number of consecutive night shifts on diurnal rhythms in cortisol, melatonin and heart rate variability (HRV): a systematic review of field studies.
abstract: OBJECTIVE
  The purpose of this review is to summarize the current knowledge from field studies on how many consecutive night shifts are required for adaptation of diurnal rhythms in cortisol, melatonin and heart rate variability (HRV) to night work.
  
  
  METHODS
  A systematic search of the databases PubMed and Web of Science resulted in 18 studies selected for review.
  
  
  RESULTS
  Cortisol was measured in five studies, melatonin in 11 studies and HRV in four studies. Diurnal rhythms were assessed by use of several different measures based on three to eight samples per day for cortisol and melatonin and 24-h recordings for HRV. Most of the studies in the review were small studies with less than 30 participants, and most studies evaluated diurnal rhythms after only two consecutive night shifts whereas only six studies used seven or more consecutive night shifts. The majority of studies found that adaptation to night work had not occurred after two consecutive night shifts, whereas a small number found evidence for full adaptation after seven consecutive night shifts based on diurnal rhythms in cortisol and melatonin.
  
  
  CONCLUSIONS
  There are methodological differences in the field studies analyzing diurnal rhythms and large diversity in the occupational fields studied. Nevertheless, we conclude that diurnal rhythms in cortisol, melatonin and HRV are not adapted to night work after 1-3 consecutive night shifts. Studies are needed to establish how many consecutive night shifts are needed for full adaptation of diurnal rhythms to night work.
authors.affiliation: Department of Public Health, University of Copenhagen, Øster Farimagsgade 5, 1014, København K, Denmark. maaj@sund.ku.dk.
authors.forename: Marie Aarrebo
authors.initials: MA
authors.lastname: Jensen
authors.affiliation: National Research Centre for the Working Environment, Lersø Parkallé 105, 2100, København Ø, Denmark.
authors.forename: Anne Helene
authors.initials: AH
authors.lastname: Garde
authors.affiliation: National Research Centre for the Working Environment, Lersø Parkallé 105, 2100, København Ø, Denmark.
authors.forename: Jesper
authors.initials: J
authors.lastname: Kristiansen
authors.affiliation: Department of Public Health, University of Copenhagen, Øster Farimagsgade 5, 1014, København K, Denmark.
authors.forename: Kirsten
authors.initials: K
authors.lastname: Nabe-Nielsen
authors.affiliation: Department of Public Health, University of Copenhagen, Øster Farimagsgade 5, 1014, København K, Denmark.|National Research Centre for the Working Environment, Lersø Parkallé 105, 2100, København Ø, Denmark.
authors.forename: Åse Marie
authors.initials: ÅM
authors.lastname: Hansen
mesh_terms: D000222:Adaptation, Physiological; D002940:Circadian Rhythm; D006339:Heart Rate / Q000502:physiology*; D006801:Humans; D006854:Hydrocortisone / Q000378:metabolism*; D008550:Melatonin / Q000378:metabolism*; D016272:Occupational Health; D013997:Time Factors; D014940:Work Schedule Tolerance / Q000502:physiology*
chemical_list: D008550:Melatonin; D006854:Hydrocortisone
keywords: Adaptation to night work; Circadian disruption; Field studies