Purpose: 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. Conclusion: 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.
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