Background: Heart rate variability (HRV) is an established, non-invasive marker used to assess autonomic recovery and training adaptation in competitive swimming. However, the impact of short-term recovery periods, such as a weekly rest day, is not well established, especially in female swimmers, who remain understudied despite potential sex-specific autonomic responses. Aim: To investigate the effect of a weekly rest day (Sunday) on HRV in highly trained female swimmers. Methods: Five female tier 3-4 swimmers (McKay et al. 2022; age: 19±5years; body mass: 59±8kg; height: 167±5cm; WA points: 711±56;) from the same swimming club were monitored across 6-7 consecutive weekly cycles (n=32 observations). Weekly training comprised 9-10 ~5.000m swim sessions and two 1-hour dryland sessions. HRV was assessed using standardized morning orthostatic tests (5min in supine and standing positions) on Fridays and Mondays (following Sunday rest) during the first macrocycle (September to December). Variables included resting HR, RMSSD, lnRMSSD, time to peak HR, peak HR, and frequency-domain indices (VLF, LF, HF). A repeated-measures ANOVA was used, including between-subject effects. Results: Significant improvements on autonomic system related variables were observed following the rest day. Resting HR decreased (59±8 vs 55±7 bpm; p<0.001, *²=0.488), while parasympathetic-related markers increased, including supine RMSSD (89.8±42.4 vs 108.7±50.4 ms; p=0.002, *²=0.297) and lnRMSSD (1.91±0.18 vs 2.00±0.17; p<0.001, *²=0.415). HF also increased in both supine (p=0.006, *²=0.250) and standing positions (p=0.026, *²=0.171). No significant changes were observed in LF or the others orthostatic reactivity markers (p>0.05). Between-subject analyses indicated consistent directional responses across all the five swimmers for resting HR and supine RMSSD and lnRMSSD, suggesting a uniform physiological adaptation to the rest day. Conclusions: A weekly rest day improved autonomic regulation, by lowering resting heart rate and increasing parasympathetic activity in highly trained female swimmers. These findings may reflect conditions supportive of maintaining athlete health and reducing the risk of maladaptation, such as overreaching or illness.
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