title: Diurnal variation in the diving bradycardia response in young men.
abstract: OBJECTIVE
  The present study aimed to examine diurnal variation of the diving bradycardia responses on the same day.
  
  
  METHODS
  Eighteen young men (age 26 ± 2 years; height 174.2 ± 6.0 cm; body mass 70.2 ± 8.1 kg; body fat 18.0 ± 3.8 %; mean ± standard deviation) participated in this study. Oral temperature, heart rate variability (HRV) from 5-min of electrocardiogram data, and diving bradycardia responses were measured at 0900, 1300, and 1700 hours daily. All participants performed diving reflex tests twice in the sitting position with the face immersed in cold water (1.9-3.1 °C) and apnea at midinspiration for a minimum of 30 s and as long as possible, in consecutive order.
  
  
  RESULTS
  Oral temperature was found to be less in the morning (0900) than in the afternoon (1300) and evening (1700). In the frequency domain parameters of heart rate variability, the natural logarithms of high-frequency power were higher in the morning than in the evening. All participants showed bradycardia response to the two diving reflex tests. The peak values of R-R interval during the diving reflex test both for as long as possible and 30 s were longer in the morning than in the afternoon and evening.
  
  
  CONCLUSIONS
  Our results indicated that the maximal bradycardia during the diving reflex test exhibits a diurnal variation, with peak levels at morning and gradual decrease towards the evening. The HRV indexes show the same variation.
authors.affiliation: Faculty of Sport Sciences, Waseda University, 2-579-15 Mikajima, Tokorozawa, Saitama, 359-1192, Japan. m.konishi@aoni.waseda.jp.
authors.forename: Masayuki
authors.initials: M
authors.lastname: Konishi
authors.affiliation: Faculty of Letters, Kokushikan University, 4-28-1 Setagaya, Setagaya-ku, Tokyo, 154-8515, Japan.
authors.forename: Hiroshi
authors.initials: H
authors.lastname: Kawano
authors.affiliation: Graduate School of Sport Sciences, Waseda University, 2-579-15 Mikajima, Tokorozawa, Saitama, 359-1192, Japan.
authors.forename: Mi
authors.initials: M
authors.lastname: Xiang
authors.affiliation: Graduate School of Sport Sciences, Waseda University, 2-579-15 Mikajima, Tokorozawa, Saitama, 359-1192, Japan.|Research Fellow of the Japan Society for the Promotion of Science, 5-3-1 Kojimachi, Chiyoda-ku, Tokyo, 102-0083, Japan.
authors.forename: Hyeon-Ki
authors.initials: HK
authors.lastname: Kim
authors.affiliation: Graduate School of Sport Sciences, Waseda University, 2-579-15 Mikajima, Tokorozawa, Saitama, 359-1192, Japan.
authors.forename: Karina
authors.initials: K
authors.lastname: Ando
authors.affiliation: Graduate School of Sport Sciences, Waseda University, 2-579-15 Mikajima, Tokorozawa, Saitama, 359-1192, Japan.
authors.forename: Hiroki
authors.initials: H
authors.lastname: Tabata
authors.affiliation: Graduate School of Sport Sciences, Waseda University, 2-579-15 Mikajima, Tokorozawa, Saitama, 359-1192, Japan.|Japan Institute of Sports Sciences, 3-15-1 Nishigaoka, Kita-ku, Tokyo, 115-0056, Japan.
authors.forename: Mio
authors.initials: M
authors.lastname: Nishimaki
authors.affiliation: Faculty of Sport Sciences, Waseda University, 2-579-15 Mikajima, Tokorozawa, Saitama, 359-1192, Japan.
authors.forename: Shizuo
authors.initials: S
authors.lastname: Sakamoto
mesh_terms: D000328:Adult; D001049:Apnea / Q000503:physiopathology; D001919:Bradycardia / Q000503:physiopathology*; D002940:Circadian Rhythm / Q000502:physiology*; D065508:Diving Reflex / Q000502:physiology*; D006339:Heart Rate / Q000502:physiology; D006801:Humans; D008297:Male
keywords: Autonomic nervous system; Bradycardia; Circadian rhythm; Vagal activity