Therefore, the present study aims to evaluate the role of the impact of altitude on sleep and the menstrual cycle in the inter- and intraindividual variability of altitude training effectiveness. In order to do so, elite female cyclists will be monitored before, during and after an altitude training camp. The monitoring will include menstrual cycle characteristics, sleep and altitude effectiveness and will start three months before the start of the altitude training camp and end two months after the altitude training camp. Both naturally cycling women and women using contraceptives will be included in the study. Menstrual cycle monitoring will take place via self-reports and via a daily saliva (Eli Health) and urine (Proov) test to measure progesterone concentration. Besides proges-terone concentration, the sampled urine will also be used to perform an ovulation test on (i.e., measuring the luteinizing hormone). In addition, a blood sample will be collected at the start of each menstrual cycle to evaluate the concentration of menstrual cycle-related hormones (e.g., fol-licle-stimulating hormone, luteinizing hormone, estrogen, and progesterone) and to evaluate the functioning of the Hypothalamic-Pituitary-Adrenal Axis (i.e., cortisol concentration). Sleep moni-toring will be performed via the use of questionnaires, actigraphy and polysomnography. Lastly, altitude effectiveness will be evaluated via the altitude-associated response in total hemoglobin mass and via an all out cycle ergometer task.

Elite athletes are constantly aiming to improve their performance and, eventually, outperform their opponents. In this endeavor, various methods have been designed to gain an advantage over the other competitors. One of those methods, which has been in use for quite some time and remains very popular, is altitude training. Currently, multiple protocols of altitude training have been developed. A general distinction can be made between "live high, train high" (LHTH), "live high, train low" (LHTL), and "live low, train high" (LLTH) altitude training. The LHTL protocol is cur-rently being put forward as the most effective one for training gains.

The overall effectiveness of altitude training to improve performance is backed up by a significant amount of scientific data. Nonetheless, up to date, the efficacy of altitude training is still questioned by some, due the lack of rigorous and well-controlled investigations, and no scientific consensus exists. Since Lundby and colleagues published their critical views on the general application of altitude training to enhance performance in elite athletes, follow-up research has substantiated these critical views of Lundby and colleagues. Nevertheless, the debate on the usefulness of altitude training in elite athletes is still ongoing, and currently revolves around whether athletes with an already high hemoglobin mass (i.e., elite athletes) can successfully increase their hemoglobin mass via altitude training.

A topic that could provide some new insights is the issue of intra- and interindividual variability in the response to altitude training, and the underlying mechanisms of these variabilities. Multiple studies have been performed that clearly outline the presence of both intra- and interindividual variability in the response to altitude training. The determination and evaluation of all state and trait-specific factors that could influence an athlete's response to altitude training is cur-rently ongoing. Nummela et al. showed that the mean effectiveness of altitude training in yielding an increase in hemoglobin mass could rise from 56% to 69% when targeting altitude exposure (2,000-2,500 m), iron deficiency and inflammation as moderating state factors. This emphasizes the need to carefully consider all the possible moderating state factors that may influence an athlete's response to altitude training. Furthermore, these findings stress the need for future research to describe more accurately how these different influencing state factors (and potential oth-er state and trait factors) interact to impact the altitude training-response in both elite and recrea-tional athletes.

One of these potentially crucial factors that could play a role in the effectiveness of altitude training to trigger performance-improving adaptations is sleep. Sleep is one of the most important aspects of recovery, and nowadays it is recommended to stay below 3,000 m (or an equivalent normobaric reduction of inspired O2) at night in altitude training paradigms. This recommendation is based on the fact that sleep is impaired at high altitude, and impaired sleep could counteract the positive physiological responses that are aimed for, certainly when it is prolonged for ~2-3 weeks (i.e., the current suggested optimal hypoxic dose, taking into account altitude and exposure time [1]). However, the guideline to stay below 3,000 m to prevent altitude-induced sleep impairments might be inadequate. Hoshikawa et al. demonstrated that acute exposure to normobaric hypoxia equivalent to a 2,000 m altitude decreased slow-wave sleep in athletes, but it did not change subjective sleepiness or amounts of urinary catecholamines. These results point out that the athlete's sleep might be disturbed even at moderate altitudes of 2,000 m and, more importantly, that athletes are not aware of it (i.e., subjective sleepiness did not change). Moreover, the study of Hoshikawa et al. also revealed that the apnea/hypopnea index (AHI; i.e., the number of signif-icant respiratory events qualifying as apnea or hypopnea per hour of sleep) increased in hypoxia compared to normoxia, and the magnitude of this effect varied widely among participants (i.e., high interindividual variability). This high interindividual variability might be associated with the interindividual variability that is observed in altitude training effectiveness. A hypothesis that is further substantiated by the recently published data of Mujika et al., that demonstrates a link between subjective sleep quality and the effectiveness of altitude training to increase total hemo-globin mass.

Specifically within female athletes, another potential crucial factor in the effectiveness of altitude training is the hypothalamic-pituitary-ovarian (HPO) axis function. The hypothalamic-pituitary-ovarian (HPO) axis regulates reproductive function, including the orchestration of ovula-tion and menstrual cyclicity. HPO axis suppression leads to altered hormonal patterns and conse-quently short luteal phases, anovulation and amenorrhoea. Shaw et al. concluded in their sys-tematic review that, if lowlanders travel to highland for short or longer duration, the high altitude-hypoxia affects their menstrual cycle more adversely than the natives. The variation in female hormones may contribute in unsuccessful ovulation, menstrual cycle, and subsequently pregnancy at high altitude. A disturbed HPO axis function at altitude can, subsequently, negatively im-pact the effectiveness of altitude training. For example, Heikura et al. recently reported lower pre-hypoxic exposure hemoglobin mass levels in amenorrheic versus eumenorrheic women, sug-gesting that menstrual dysfunction, an indicator of long-term low energy availability, may influence the altitude exposure-related increase in hemoglobin mass or its magnitude.

Therefore, the present study aims to evaluate the role of the impact of altitude on sleep and the menstrual cycle in the inter- and intraindividual variability of altitude training effectiveness. In order to do so, elite female cyclists will be monitored before, during and after an altitude training camp. The monitoring will include menstrual cycle characteristics, sleep and altitude effectiveness and will start three months before the start of the altitude training camp and end two months after the altitude training camp. Both naturally cycling women and women using contraceptives will be included in the study. Menstrual cycle monitoring will take place via self-reports and via a daily saliva (Eli Health) and urine (Proov) test to measure progesterone concentration. Besides proges-terone concentration, the sampled urine will also be used to perform an ovulation test on (i.e., measuring the luteinizing hormone). In addition, a blood sample will be collected at the start of each menstrual cycle to evaluate the concentration of menstrual cycle-related hormones (e.g., follicle-stimulating hormone, luteinizing hormone, estrogen, and progesterone) and to evaluate the functioning of the Hypothalamic-Pituitary-Adrenal Axis (i.e., cortisol concentration). Sleep monitoring will be performed via the use of questionnaires, actigraphy and polysomnography. Lastly, altitude effectiveness will be evaluated via the altitude-associated response in total hemoglobin mass and via an all out cycle ergometer task.

Inclusion Criteria:

• Healthy
• Female
• Elite cyclist

Exclusion Criteria:

-

Eligibility criteria are being a healthy female elite cyclist. Both naturally cycling women and women who use contraception are eligible.

Given that we will be working with elite cyclists, the IPD will be confidential.