title: Influence of nocturnal and daytime sleep on initial orthostatic hypotension.
abstract: OBJECTIVE
  The incidence of vasovagal syncope is more common in the morning. Previous researchers have reported negligible diurnal variation in the physiological responses associated with initial orthostatic hypotension (IOH). Nevertheless, physical activity and sleep prior to morning and afternoon test times have not been controlled and may influence the findings. We designed a semi-constant routine protocol to examine diurnal variation in cardiorespiratory and cerebrovascular responses to active standing.
  
  
  METHODS
  At 06:00 and 16:00 hours, nine males (27 ± 9 years) completed an upright-stand protocol. Altimetry-measured sleep durations were 3.3 ± 0.4 and 3.2 ± 0.6 h immediately prior to the morning and afternoon test times. Continuous beat-to-beat measurements of middle cerebral artery velocity (MCAv), mean arterial blood pressure (MAP), heart rate (HR), and end-tidal carbon dioxide were obtained. Intestinal body temperature and salivary melatonin concentrations were also measured.
  
  
  RESULTS
  Compared with the afternoon, resting HR and body temperature were 4 ± 2 beats min(-1) and 0.45 ± 0.2 °C lower, respectively, whereas melatonin concentration was 28.7 ± 3.2 pg ml(-1) higher in the morning (P ≤ 0.02). Although all individuals experienced IOH at both times of the day, the initial decline in MAP during standing was 13 ± 4 mmHg greater in the afternoon (P = 0.01). Nevertheless, the decline in MCAv was comparable at both times of day (mean difference: 2 ± 3 cm s(-1); P = 0.5).
  
  
  CONCLUSIONS
  These findings indicate that a bout of sleep in the afternoon in healthy young individuals results in greater IOH that is compensated for by effective cerebral blood flow regulation.
authors.affiliation: School of Health and Exercise Sciences, Faculty of Health and Social Development, University of British Columbia Okanagan, 111 Health Science Centre, 1088 Discovery Avenue, Kelowna, BC, V1V 1V7, Canada, nia.lewis@ubc.ca.
authors.forename: N C S
authors.initials: NC
authors.lastname: Lewis
authors.forename: H
authors.initials: H
authors.lastname: Jones
authors.forename: P N
authors.initials: PN
authors.lastname: Ainslie
authors.forename: A
authors.initials: A
authors.lastname: Thompson
authors.forename: K
authors.initials: K
authors.lastname: Marrin
authors.forename: G
authors.initials: G
authors.lastname: Atkinson
mesh_terms: D000328:Adult; D001794:Blood Pressure; D001831:Body Temperature; D002560:Cerebrovascular Circulation; D006339:Heart Rate; D006801:Humans; D007024:Hypotension, Orthostatic / Q000503:physiopathology*; D008297:Male; D008550:Melatonin / Q000378:metabolism; D017440:Photoperiod; D012890:Sleep
chemical_list: D008550:Melatonin