Purpose: The aim of this study was to investigate within-cycle differences in nocturnal heart rate (HR) and heart rate variability (HRV) in naturally menstruating women (NM) and women using combined hormonal contraceptives (CU) or progestin-only hormonal contraceptives (PU). Methods: Physically active participants were recruited into three groups: NM (n = 19), CU (n = 11), and PU (n = 12). Participants' HR and HRV (with Bodyguard 2 HRV monitor) and blood hormones were monitored during one menstrual cycle (MC) (NM group) or for 4 wk (CU and PU groups). Estradiol, progesterone, and luteinizing hormone were analyzed from fasting blood samples collected four times in the NM (M1 = bleeding, M2 = follicular phase, M3 = ovulation, and M4 = luteal phase) and PU groups (M1 = lowest E 2 , M2 = M1 + 7 d, M3 = M1 + 14 d, and M4 = M1 + 21 d) and twice in the CU group (active and inactive pill phases). After every blood sample, nightly HR and HRV were recorded and examined as an average from two nights. Results: Hormonal concentrations differed (P < 0.05) between MC phases in the NM and PU groups, but not (P ≥ 0.116) between the active and the inactive phases in the CU group. In the NM and PU groups, some of the HRV values were higher, whereas in the NM group, HR was lower during M2 compared with M3 (P < 0.049) and M4 (P < 0.035). In the CU group, HRV values (P = 0.014-0.038) were higher, and HR was lower (P = 0.038) in the inactive phase compared with the first week of the active phase. Conclusions: The MC and the hormonal cycle phases influence autonomic nervous system balance, which is reflected in measurements of nocturnal HR and HRV. This should be considered when monitoring recovery in physically active individuals.

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Purpose To investigate whether 4 weeks of normobaric “live high–train low and high” (LHTLH) causes different hematological, cardiorespiratory, and sea‐level performance changes compared to living and training in normoxia during a preparation season. Methods Nineteen (13 women, 6 men) cross‐country skiers competing at the national or international level completed a 28‐day period (∼18 h day−1) of LHTLH in normobaric hypoxia of ∼2400 m (LHTLH group) including two 1 h low‐intensity training sessions per week in normobaric hypoxia of 2500 m while continuing their normal training program in normoxia. Hemoglobin mass (Hbmass) was assessed using a carbon monoxide rebreathing method. Time to exhaustion (TTE) and maximal oxygen uptake (VO2max) were measured using an incremental treadmill test. Measurements were completed at baseline and within 3 days after LHTLH. The control group skiers (CON) (seven women, eight men) performed the same tests while living and training in normoxia with ∼4 weeks between the tests. Results Hbmass in LHTLH increased 4.2 ± 1.7% from 772 ± 213 g (11.7 ± 1.4 g kg−1) to 805 ± 226 g (12.5 ± 1.6 g kg−1) (p < 0.001) while it was unchanged in CON (p = 0.21). TTE improved during the study regardless of the group (3.3 ± 3.4% in LHTLH; 4.3 ± 4.8% in CON, p < 0.001). VO2max did not increase in LHTLH (61.2 ± 8.7 mL kg−1 min−1 vs. 62.1 ± 7.6 mL kg−1 min−1, p = 0.36) while a significant increase was detected in CON (61.3 ± 8.0–64.0 ± 8.1 mL kg−1 min−1, p < 0.001). Conclusions Four‐week normobaric LHTLH was beneficial for increasing Hbmass but did not support the short‐term development of maximal endurance performance and VO2max when compared to the athletes who lived and trained in normoxia.

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