Sleep, Nutrition, and Injury Risk in Adolescent Athletes: A Narrative Review

narrative review explores the impact of sleep and nutrition on injury risk in adolescent athletes. Sleep is viewed as essential to the recuperation process and is distinguished as an active participant in recovery through its involvement in growth, repair, regeneration, and immunity. Fur- thermore, the literature has shown that the sleep of athletes impacts elements of athletic performance including both physical and cognitive performance, recovery, injury risk, and mental well-being. For sleep to have a restorative effect on the body, it must meet an individual's sleep needs whilst also lasting for an adequate duration and being of adequate quality, which is age-dependent. The literature has suggested that athletes have increased sleep needs compared to those of the general population and thus the standard recommendations may not be suf cient for athletic populations. Therefore, a more individualised approach accounting for overall sleep health may be more appropri- ate for addressing sleep needs in

adequate duration and being of adequate quality, which is age-dependent. The literature has suggested that athletes have increased sleep needs compared to those of the general population and thus the standard recommendations may not be suf cient for athletic populations. Therefore, a more individualised approach accounting for overall sleep health may be more appropri- ate for addressing sleep needs in individuals including athletes. The literature has demonstrated that adolescent athletes achieve, on average, ~6.3 h of sleep, demonstrating a discrepancy between sleep recommendations (8–10 h) and actual sleep achieved. Sleep–wake cycles undergo development dur- ing adolescence whereby adaptation occurs in sleep regulation during this phase. These adaptations increase sleep pressure tolerance and are driven by the maturation of physiological, psychological, and cognitive functioning along with delays in circadian rhythmicity, thus creating an environment for inadequate sleep during adolescence. As such, the adolescent period is a phase of rapid growth and maturation that presents multiple challenges to both sleep and nutrition; consequently, this places a signi cant burden on an adolescent athletes' ability to recover, thus increasing the likelihood of injury. Therefore, this article aims to provide a comprehensive review of the available literature on the importance of sleep and nutrition interactions in injury risk in adolescent athletes. Furthermore, it provides foundations for informing further investigations exploring the relation of sleep and nutrition interactions to recovery during adolescence. Keywords:adolescent; athletes; sleep; nutrition; injury risk 1. Introduction Sleep can be de ned as a reversible behavioural state of impaired consciousness through a reduction in sensory and motor activity [1–5]. Sleep is regarded as an ac- tive regulatory process [6] and facilitates the proper functioning of the brain and cogni- tive performance while also regulating physiological functions including substrate and energy metabolism [7–9], cardiovascular function [10,11], appetite [12], endocrine func- tion [13,14], and immune function [15]. Several facilitating theories for sleep have been Nutrients2023,15, 5101.

Nutrients2023,15, 5101 2 of 16 hypothesised [5,16] , including the regeneration of immune and endocrine function, the ner- vous system, and metabolic cost of living, and aids in cognitive development and synaptic plasticity [17]. As such, sleep is viewed as essential to the recuperation process [3,18] and is distinguished as an active participant in recovery through its involvement in growth, repair, regeneration, and immunity [5]. Furthermore, the literature has shown that the sleep of athletes impacts elements of athletic performance including both physical and cognitive performance, recovery, injury risk, and mental well-being [4,19–23]. The architecture of sleep is organised into multiple 90-min series of one rapid-eye- movement (REM) and three non-REM sleep phases (N1, N2, and N3) [24–26]. Non-REM and REM sleep are distinct in nature and are characterised by brainwave activity, eye move- ment, cardiac rhythm, muscle tone, breath rate, and arousal thresholds [27,28]. REM sleep facilitates neurological regeneration, learning, memory, and emotionalregulation [27,29,30] . Non-REM sleep is a three-phased process whereby the propensity to wake (the arousal threshold) is lowest during the rst phase and progressively increases through to the highest point in the nal phase (N3) [27,28] and can be characterised by the wave activity of the brain [27,31]. Non-REM sleep functions to support the regeneration of the nervous system, conserve energy, release anabolic hormones that augment protein synthesis to facilitate muscle recovery [32,33], and mobilise free fatty acids for ATP production [34]. For a detailed breakdown of the brainwave characteristics of the sleep cycle, readers should refer to the AASM manual for the scoring of sleep and associated events [25], and the principles and practices of sleep medicine [27]. 2. Importance of Sleep Health Sleep health is de ned as an individualised and context-speci c multidimensional pattern of sleep and wakefulness that supports physical and mental well-being [35] and is an integral component of not only living a healthy lifestyle [36,37] but also of adaptation and recovery [3,27,32,38,39]. The relationship between sleep and recovery in athletes can be viewed in terms of three key factors that affect restoration processes: 1. sleep duration (total sleep requirements including

pattern of sleep and wakefulness that supports physical and mental well-being [35] and is an integral component of not only living a healthy lifestyle [36,37] but also of adaptation and recovery [3,27,32,38,39]. The relationship between sleep and recovery in athletes can be viewed in terms of three key factors that affect restoration processes: 1. sleep duration (total sleep requirements including napping); 2. sleep quality (total sleep absent of sleep disorders, environmental disturbances, or sleep fragmentation); 3. sleep phase (circadian timing of sleep during the light–dark cycle) [39,40]. During adolescence, the psychosocial and societal pressures experienced may result in adverse sleep health and reduced recovery capacity [41,42]. This is due to sleep de ciencies, which have a negative impact on health and are linked to increases in all-cause mortality and disease risk [43]. Sleep deprivation (an insuf cient sleep duration compared to the basal level) and disturbances (the inability to initiate and/or maintain the sleep–cycle) are also risk factors for adverse health, recovery, and injury risk in athletic populations [39,40,44–48]. For sleep to have a restorative effect on the body, it must meet an individual's sleep needs whilst also lasting for an adequate duration and being of adequate quality, which is age-dependent [46]. Sleep needs can be de ned as the optimum quantity of sleep required to maintain alertness and function throughout daily living [5]. Sleep duration can be in u- enced by exogenous and endogenous environmental characteristics, which adds complexity to de ning an “optimal” sleep pattern due to high inter-individual differences [49–51]. Peripheral tissues contain molecular clocks within each cell that dictate the expression of clock-controlled genes in a period (the required time for a cortical neuron oscillation) or phase (waking time in relation to the light cycle) [49,52]. Processes controlled by circadian rhythms also in uence sleep duration; thus, the time at which sleep occurs in the light cycle also has an integral role in sleep duration [50,53,54]. The difference between sleep needs and actual sleep duration is known as sleep debt [5]. 3. Sleep Adaptations during Adolescence The chronotype that de nes the expression of individual circadian

Processes controlled by circadian rhythms also in uence sleep duration; thus, the time at which sleep occurs in the light cycle also has an integral role in sleep duration [50,53,54]. The difference between sleep needs and actual sleep duration is known as sleep debt [5]. 3. Sleep Adaptations during Adolescence The chronotype that de nes the expression of individual circadian rhythmicity [55] may similarly shift during adolescence [56–60] due to the greater robustness to increased sleep pressure [61] and environmental factors that increase evening alertness [42]. An

Nutrients2023,15, 5101 3 of 16 individual's chronotype is mainly dictated by their genetic makeup; however, environ- mental and societal factors also affect the chronotype [55,62]. Cross-sectional research has evidenced that during adolescence, the distribution of the chronotype shifts toward the evening chronotype, reverting back to the earlier chronotypes post-maturation [56–60]. As sleep needs change over the lifespan, The National Sleep Foundation has published guidelines for age-dependent sleep durations, which includes recommendations for the fol- lowing: adolescents (8–10 h), adults (7–9 h), and older adults (7–8 h) [46]. The literature has suggested that athletes have increased sleep needs, and thus the general recommendations may not be suf cient for athletic populations [23,63]. Therefore, a more individualised approach accounting for overall sleep health may be more appropriate for addressing sleep needs in individuals including athletes [23,35,63]. To feel rested, the literature has demonstrated that elite athletes need ~8.3 h of sleep [64,65]. Moreover, adolescent athletes achieve, on average, ~6.3 h [47,66,67] of sleep, demonstrating a discrepancy between sleep recommendations and actual sleep achieved [42,68,69]. Sleep–wake cycles also undergo development during adolescence whereby adaptation occurs in sleep regulation during this phase [68]. These adaptations to sleep regulation increase sleep pressure tolerance (re- duced adenosine accumulation) [70–72] and are driven by the maturation of physiological, psychological, and cognitive functioning along with delays in circadian rhythmicity [73], thus creating an environment for inadequate sleep in uenced by external factors associated with adolescence (Figure) [ 41,42,73,74]. This results in ever-decreasing time spent asleep during the ages of 15–18 years, with research reporting a decrease of ~1.5–3 h during this period [70,75,76]. Despite this, an adolescents sleep needs (pressure dissipation) under free living conditions does not appear to alter from the recommended ~9.25 h [42], irrespec- tive of maturation status [77–79], thus attributing the decline to environmental factors as opposed to biological factors [73,80].Nutrients 2023, 15, x FOR PEER REVIEW 3 of 17 3. Sleep Adaptations during Adolescence The chronotype that defines the expression of individual circadian rhythmicity [55] may similarly shift during adolescence [56–60] due to the greater robustness to increased sleep pressure [61] and environmental

of maturation status [77–79], thus attributing the decline to environmental factors as opposed to biological factors [73,80].Nutrients 2023, 15, x FOR PEER REVIEW 3 of 17 3. Sleep Adaptations during Adolescence The chronotype that defines the expression of individual circadian rhythmicity [55] may similarly shift during adolescence [56–60] due to the greater robustness to increased sleep pressure [61] and environmental factors that increase evening alertness [42]. An in- dividual’s chronotype is mainly dictated by their genetic makeup; however, environmen- tal and societal factors also affect the chronotype [55,62]. Cross-sectional research has ev- idenced that during adolescence, the distribution of the chronotype shifts toward the evening chronotype, reverting back to the earlier chronotypes post-maturation [56–60]. As sleep needs change over the lifespan, The National Sleep Foundation has published guidelines for age-dependent sleep durations, which includes recommendations for the following: adolescents (8–10 h), adults (7–9 h), and older adults (7–8 h) [46]. The literature has suggested that athletes have increased sleep needs, and thus the general recommen- dations may not be sufficient for athletic populations [23,63]. Therefore, a more individu- alised approach accounting for overall sleep health may be more appropriate for address- ing sleep needs in individuals including athletes [23,35,63]. To feel rested, the literature has demonstrated that elite athletes need ~8.3 h of sleep [64,65]. Moreover, adolescent ath- letes achieve, on average, ~6.3 h [47,66,67] of sleep, demonstrating a discrepancy between sleep recommendations and actual sleep achieved [42,68,69]. Sleep–wake cycles also un- dergo development during adolescence whereby adaptation occurs in sleep regulation during this phase [68]. These adaptations to sleep regulation increase sleep pressure tol- erance (reduced adenosine accumulation) [70–72] and are driven by the maturation of physiological, psychological, and cognitive functioning along with delays in circadian rhythmicity [73], thus creating an environment for inadequate sleep influenced by exter- nal factors associated with adolescence (Figure 1) [41,42,73,74]. This results in ever-de- creasing time spent asleep during the ages of 15–18 years, with research reporting a de- crease of ~1.5–3 h during this period [70,75,76]. Despite this, an adolescents sleep needs (pressure dissipation) under free living conditions does not appear to

creating an environment for inadequate sleep influenced by exter- nal factors associated with adolescence (Figure 1) [41,42,73,74]. This results in ever-de- creasing time spent asleep during the ages of 15–18 years, with research reporting a de- crease of ~1.5–3 h during this period [70,75,76]. Despite this, an adolescents sleep needs (pressure dissipation) under free living conditions does not appear to alter from the rec- ommended ~9.25 h [42], irrespective of maturation status [77–79], thus attributing the de- cline to environmental factors as opposed to biological factors [73,80]. Figure 1. The perfect storm model adapted from [41,42]. Figure 1.The perfect storm model adapted from [41,42]. 4. Growth, Maturation, and Energy Demands in Adolescent Athletes Adolescence is the transitional life stage where the process of maturation occurs [81]. Maturation signi es the progressive period toward the adult or mature state [82], and is characterised by status (maturity state at the time of observation), timing (biological age at which speci c maturational events occur), and tempo (the rate of maturational progression) [82–86]. During the maturation period, approximately 20% of the nal adult

Nutrients2023,15, 5101 4 of 16 height is reached and 50% of the predicted adult body weight is achieved with an increase of up to 40% in bone mass [81,87]. The adolescence period is a signi cant life stage that begins in conjunction with the onset of puberty. Puberty is unique to the individual with a vast range of inter-individual differences in maturation status impacting both physical and psycho-social development [82,84,86]. As evidenced, maturation status in uences the development of and improvement in locomotive competencies in both a linear (accrual of strength capabilities) and non-linear (accrual of co-ordinative capabilities) fashion [88], in uencing training adaptations in adolescent athletes [88–91]. Puberty onset is the catalyst of the growth period where the body undergoes meaningful physical and psycho-social adaptations such as alterations to body composition, including the accrual of bone, muscle, and fat mass, metabolic and endocrine system development, the development and mat- uration of the organ system, the establishment of nutrient storage and partitioning, and the establishment of self-esteem and psychological well-being, all of which affect general health and well-being [81,92–94]. Maturation results from the outcome of a multitude of complex processes that are governed by genetics, the endocrine system, environmental constraints, and nutrient intake [86,89,95,96]. 5. Differences between Adolescent and Adult Athletes During adolescence, there are vast inter-individual differences in nutritional needs dependent on factors including maturation status, body composition, physical activity, chronological age, and gender [92,95–98]. The implementation of correct and individualised nutrition for adolescent athletes not only supports overall health, adaptation, recovery, and athletic performance, but is also necessary for meeting growth and development demands (Table) [ 92,95–98]. Compared to their adult counterparts, adolescents possess several dif- ferences in substrate storage and metabolism, in conjunction with numerous physiological and metabolic alterations associated with maturation that contribute to an individual's nutrient requirements [96]. Furthermore, nutrient and energy requirements in adolescence are also largely dictated by the interplay between three main factors:(1) currentanthro- pometry, (2) maturation state, rate, and timing, and (3) physical activity and sporting demands [92,96,97]. These energy requirements of an individual are ful lled by the intake of

metabolic alterations associated with maturation that contribute to an individual's nutrient requirements [96]. Furthermore, nutrient and energy requirements in adolescence are also largely dictated by the interplay between three main factors:(1) currentanthro- pometry, (2) maturation state, rate, and timing, and (3) physical activity and sporting demands [92,96,97]. These energy requirements of an individual are ful lled by the intake of energy-yielding macro-nutrients, carbohydrates, protein, and fat [96,99–101]. The spe- ci c energy intake of adolescent athletes should be largely dictated by total daily energy expenditure (TDEE) [96,102–104].

Nutrients2023,15, 5101 5 of 16 Table 1.Anatomical, physiological, and metabolic differences between adolescent and adult athletes adapted from [96]. Summary of Main Physiological and Metabolic Issues Surrounding Growth and Maturation Potential Consequences of These Differences in Physiology and Metabolism on Nutritional Recommendations Greater Energy Cost of Movement Children and adolescents have a higher (relative) energy cost of movement compared with that of adults. This may be due to increased stride frequency, a greater surface area:volume ratio, a more distal distribution of mass in the legs, or greater levels of contraction of the antagonist leg muscles while moving [105,106]. Increased (relative) energy requirements for physical activity need to be accounted for. Reduced Glycogen Storage Capacity Children and adolescents have a lower endogenous glycogen storage capacity compared with that of adults [107]. Reduced emphasis for young athletes to have a carbohydrate load before training/competition. Reduced Glycolytic Capabilities Children and adolescents have reduced glycolytic capabilities, with full anaerobic capabilities developing towards the end of puberty [108]. As a result, children and adolescents have lower levels of lactate production than those of adults during high-intensity exercise of the same relative intensity [107,109]. Reduced requirement for the use of buffering agents with young athletes, particularly those in pre- and peri-puberty stages. Higher Rates of Aerobic Metabolism Higher rates of aerobic metabolism exist in children during exercise. Fat oxidation rates during submaximal exercise (of the same relative intensity) are greater in children and adolescents compared with that in adults. Less mature children have a greater reliance on fat as a fuel compared with more mature adolescents. It has been suggested that these higher fat oxidation rates in children compared with those in adults are the result of lower endogenous carbohydrate stores and reduced glycolytic capabilities [110]. Young athletes may not require the same relative amount of carbohydrate as adult athletes do; however, there is a lack of evidence to support this. Further research is warranted. Greater Reliance on Exogenous Carbohydrate Children and adolescents have greater reliance on exogenous carbohydrate as a fuel source. During exercise, exogenous carbohydrate is a greater contributor to total energy supply in

Young athletes may not require the same relative amount of carbohydrate as adult athletes do; however, there is a lack of evidence to support this. Further research is warranted. Greater Reliance on Exogenous Carbohydrate Children and adolescents have greater reliance on exogenous carbohydrate as a fuel source. During exercise, exogenous carbohydrate is a greater contributor to total energy supply in children and adolescents compared with adults [110]. Exogenous carbohydrate oxidation rates are higher in less mature boys compared with more mature boys of the same chronological age; however, this is not the case in females [111,112]. Exogenous carbohydrate should be consumed during moderate-/high intensity exercise lasting longer than 60 min. Thermoregulatory Differences Children and adolescents have a larger surface area:body mass ratio [113], so, consequently, they gain and lose more heat from the environment through conduction, convection, and radiation. Adolescents who undertake regular exercise do adapt, however, improving their ability to thermoregulate through enhanced peripheral vasodilatation [114]. Regular consumption of cold avoured uids during exercise Reduced Sweating Capacity Children and adolescents have a lower sweating capacity compared with that of adults and therefore a reduced ability to lose sweat through sweat evaporation. As children mature, so too do their thermoregulation mechanisms (particularly their ability to sweat); however, these are not fully developed until late puberty [115]. Regular consumption of cold avoured uids during exercise. There is no evidence to suggest that uid requirements in young athletes are less than those of their adult counterparts, despite reduced sweat rates. Growth and Increase in Body Size Macronutrient requirements are often prescribed relative to body mass (i.e., grams per kilo, g/kg) to account for individual differences in size among young athletes. Although fat mass does not seem to signi cantly change throughout growth and maturation in young athletes, increases in body mass are primarily derived from an increase in fat-free mass [116]. An increase in stature is the result of skeletal growth and the laying down of bone mineral content (i.e., skeletal tissue). Around 95% of adult bone mineral content is achieved by the end of adolescence, with ~26% of this being accrued

The article reviews literature on sleep and nutrition's role in injury risk for adolescent athletes.