The human diving reflex is an innate cardiorespiratory adjustment triggered during apnea that concurrently activates both sympathetic and parasympathetic branches of the autonomic nervous system to maintain physiological stability under stress. Although pupil dilation and constriction are antagonistically regulated by these branches, the effect of the diving reflex on pupil diameter oscillations, known as hippus, remains unexplored. Here, we compared hippus in healthy participants during breathing or apnea either with (Wet) or without (Dry) facial immersion in cold water. In both apneic conditions, hippus exhibited reduced power in the low-frequency band (< 0.25 Hz). Notably, during Wet apnea, we observed a reallocation of power towards higher frequencies (> 0.25 Hz) and increased entropy of fluctuations, indicating a shift in autonomic balance and greater signal complexity during the diving reflex. This pilot study highlights pupil dynamics as a sensitive and non-invasive probe of autonomic adjustments underlying the human diving reflex.

Background: Heart rate variability (HRV) monitoring during real-world closed-circuit rebreather (CCR) diving remains methodologically challenging. This exploratory case report describes phase-related HRV variations obtained from a wearable device during a military CCR dive. Methods: A trained male military diver underwent continuous monitoring with a RootiRx^® single-lead wearable device placed in a custom waterproof enclosure. Three consecutive 20 min segments were analyzed: pre-dive, CCR dive, and immediate post-dive. The post-dive segment was not acquired under standardized resting conditions, and the participant continued to wear the device throughout this period. RR interval data were analyzed using Kubios HRV Scientific 4.3.0 with medium-threshold artifact correction, smoothness-prior detrending (λ = 500), and 4 Hz interpolation. Time-domain, frequency-domain, and nonlinear HRV indices were evaluated descriptively. Results: Mean heart rate was 88.1 beats/min pre-dive, 102.8 beats/min during the dive, and 86.4 beats/min post-dive. SDNN values were 35.3, 29.9, and 49.2 ms, while RMSSD values were 29.7, 24.6, and 40.3 ms, respectively. The PNS index was −1.44, −2.08, and −1.10, and the SNS index was 1.66, 2.98, and 1.26 across the three phases. LF/HF values were 1.77, 1.46, and 3.92, respectively. Automatically corrected beats represented approximately 0.34%, 0.54%, and 0.46% of beats, with effective analyzable data proportions exceeding 99.7% in all three segments. Conclusions: An analyzable RR interval series was obtained across the three operational phases in this individual recording. However, the single-subject uncontrolled design, non-standardized recording conditions, multiple confounding factors, and absence of independent underwater validation prevent causal attribution of the observed differences to immersion, CCR exposure, or autonomic adaptation. Further controlled studies using validated underwater ECG systems are required.