Behind the apparent simplicity of going to bed and falling into the arms of Morpheus lies a complex organization leading to sleep. As described by J. Allan Hobson (1989, 2005), sleep is not simply the absence of waking, but a dynamic behaviour with a special activity of the brain, controlled by elaborate and precise systems in and for the brain and the whole body. Although why we sleep remains elusive and a scientific enigma (Frank, 2006; Krueger et al., 2016), sleep is defined as a non-negotiable biological state necessary to sustain human life, alongside air, food and water, to which humans devote around one-third of their lives. Sleep is crucial in proper biopsychosocial development, and for physical and mental health, cognitive functioning, and good quality of life (Grandner and Fernandez, 2021). Nevertheless, insufficient sleep is currently one of the most commonly reported health complaints, and is associated with a range of adverse effects on health (Grandner, 2022). In this regard, the American Academy of Sleep Medicine and the Sleep Research Society urge that sleep is a priority public health and societal issue (Luyster et al., 2012; Buysse, 2014). In elite sport, the importance of sleep assumes another dimension. Practicing sport at the highest level entails highly demanding physiological and psychological requirements to succeed internationally, amidst dealing with various external stressors (e.g., travel, media, career pressure) and personal life constraints. This requires effective management of the recovery-stress balance to maintain the athlete's physical and mental health, well-being, and performance (Kenttä and Hassmén, 1998; Kellmann et al., 2018). Sleep is increasingly recognized as the foundation of athlete recovery and a crucial component in preparing for competition (Halson and Juliff, 2017; O`Donnell et al., 2018a). It is currently receiving particular interest from athletic/sporting organizations and the scientific community (Venter, 2014; Lastella et al., 2020a). In this context, numerous studies have sought to characterize the sleep of elite athletes, whether during training phases or competitions (Gupta et al., 2017; Roberts et al., 2019b). The picture is not rosy: the prevalence of sleep complaints and inadequate sleep has been reported to be high in the elite athlete population. Frequent travel, unfamiliar sleeping environments, varied training and competition schedules, and social and personal life obligations (e.g., family, work, study commitments) are well-identified contributory factors to sleep disturbances in athletes (Walsh et al., 2020). Among these factors, the substantially high training load imposed to prepare for major competitions is a potential disruptor of sleep in elite athletes that is not yet fully understood. A dichotomy still remains. On the one hand, practicing regular physical exercise is recommended by the World Health Organization for improving both the quantity and quality of sleep in the general population (Bull et al., 2020). Numerous studies have demonstrated its benefits in improving the quantity and quality of sleep by reducing nocturnal awakenings, the time to fall asleep and the internal structure of sleep (architecture) towards a deeper sleep (Uchida et al., 2012; Kredlow et al., 2015; Park et al., 2021). On the other hand, some studies report sleep disturbances in elite athletes when exposed to excessive training loads (Roberts et al., 2019b; Walsh et al., 2020). Maladaptive response to training, i.e., prolonged decline in performance and/or increased perceived fatigue leading to overreaching/overtraining state, is usually accompanied by sleep complaints (Kellmann, 2010; Meeusen et al., 2013; Schwellnus et al., 2016). The vast majority of studies that impose large increases in training load have described experiencing more restless sleep, accompanied by a reduction in both sleep quantity and efficiency (i.e., time asleep as a percentage of time in bed) (Roberts et al., 2019b). Nonetheless, the literature is limited, with most information from studies using subjective or actigraphy measures for assessing sleep quantity and quality. Such methods do not provide the intricate neurophysiological features and internal structure of sleep typically achieved through polysomnography, which encompasses crucial information regarding sleep quality (McCarter et al., 2022). Using such an approach would provide insight into how athletes` sleep architecture responds to varying training loads. In this view, increased training load leads to physiological and psychological stress and may disrupt the organization and life around training (e.g., increased number and duration of training sessions), which can affect sleep. However, their role in the potential detrimental effect of an overload on athletes` sleep remains unknown. While such investigations conducted in real-life settings may face various logistical and operational challenges, in particular when performed in elite sport setup, they would definitely contribute to a better understanding of how training load affects sleep and whether cofounding factors are involved. This could offer guidance for practitioners in managing training programs and loads for the benefit of athletes' sleep, well-being, and performance. At a time when sleep is a matter of health and society, developing practical strategies that can enhance it is highly necessary considering its importance in performance, well-being and health (Walsh et al., 2020). Several pharmacological and non-pharmacological sleep-enhancing strategies have been studied. Some are considered effective for improving sleep, such as cognitive-behavioural therapy, sleep education and hygiene, and dietary manipulation (Pedlar et al., 2023). However, they have the limitation of requiring individual engagement, may not be short-term effective solutions, nor practical in high-pressure situations that occur in elite sport setting. Bedding is our nightly support for sleep, and is largely little studied. Due to the close, reciprocal relationship between sleep and the body's temperature regulation systems (Van Someren, 2006; Krauchi and Deboer, 2010; Harding et al., 2019), the thermal properties of bedding have been identified as a key factor of sleep quality (Troynikov et al., 2018). In this context, recent works have investigated the benefits of a high-heat-capacity bedding system on sleep in healthy volunteers (Kräuchi et al., 2018; Herberger et al., 2020, 2024) or elite rugby union players (Aloulou et al., 2020b). Sleeping on this bedding, consisting of a foam mattress coated with a gel sheet on top, has been shown to enhance the slow and continuous removal of body heat during sleep through conductive heat transfer. This improvement in heat dissipation has been associated with enhanced sleep quality and subjective sleep comfort. Nevertheless, these studies were conducted from an acute perspective, i.e., a single night. The putative relevance of this bedding strategy when used chronically during a typical training context in athletes is currently unknown. There is also no evidence that this strategy can mitigate the potential detrimental effect of high training load on sleep. Furthermore, increasing and intensified exposure to high temperatures due to climate change is a major current issue (Patz et al., 2014; Watts et al., 2018) affecting both the general population and elite athletes. Sleep quantity and quality are highly sensitive to night-time ambient temperature (Lan et al., 2017), to the extent that predictions suggest that by 2099, rising temperature may erode 50-58 h of sleep per person-year (Minor et al., 2022). Given its interesting thermal properties, investigating the effectiveness of a high-heat-capacity bedding system in athletes exposed to elevated temperatures is part of the effort to sustainably preserve global public health and human well-being. Additionally, given the inter-individual variability of sleep (Nedelec et al., 2018) and the adaptive response to heat (Racinais et al., 2012; Corbett et al., 2018), describing individual responses to this strategy in this context is particularly relevant for achieving more precise guidance in prescribing individualized sleep interventions (Fullagar and Bartlett, 2016). The aim of this doctoral thesis is to deepen our understanding of the impact of training load on sleep and to explore the effectiveness of a high-heat-capacity bedding system on athletes' sleep within a typical training context. Special attention will be paid on assessing sleep architecture and the use of objective and subjective sleep monitoring methods to encompass the concepts of sleep quantity and quality. The experimental part of this work is structured around three studies conducted in situ, within athletes' training contexts, utilizing descriptive, controlled, and interventional designs. This manuscript consists of four main chapters. The first chapter includes a literature review on the importance of sleep for athletes and the role of training load as a potential modulator of sleep. We will also explore the relationship between sleep and the body's temperature regulation systems, as well as studies investigating thermal strategies to influence sleep. The second chapter will provide detailed information on the general methodology associated with experimental studies. The third chapter will focus on the experimental contributions and present the findings of the three studies conducted within the framework of this PhD thesis. Finally, the fourth and last chapter will discuss the overall results, leading to research perspectives and practical applications of the present work.
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