How Sleep Affects Recovery and Performance in Basketball: A Systematic Review

Background: Sleep is considered an essential component related to physiological and psychological recovery in athletes and particularly in basketball, given the impact of condensed travel and game schedules on player health and performance. Objective: The aim of this systematic review is to examine studies published to date on sleep and basketball performance. Methodology: All scientific articles that reported a relationship between sleep and its possible impact on performance in basketball are included. The research processes followed the PRISMA criteria, and the relevant articles were extracted (PubMed, WOS, Scopus) as of December 31, 2021. Results: Twenty-eight articles were selected for inclusion and data extraction, with 27 demonstrating that sleep is a vital component in the recovery of basketball players and their corresponding on-court performance. Three central themes that we identified: (1) the quality and extension of sleep (the better quality and more extension of sleep, better performance and lower probability of injury); (2) influence of the players circadian rhythm (travel and game scheduling do not currently facilitate or take this into account); and (3) higher training loads and/or increased stress may jeopardize a subsequent good night’s sleep, which should be taken into account when scheduling practices and workouts. Conclusion: The current systematic review regarding sleep and basketball performance and highlights that there is a strong relationship between both variables. Collectively, the evidence supports the critical influence of sleep on player recovery and basketball performance and risk for injury.


Introduction
Basketball is an intermittent sport made up of high intensity repeated accelerations, decelerations, change of direction and jump landings, which places a high demand on players, both physically and cognitively [1]. In this team sport, the main physical actions that determine performance are accelerations and decelerations, changes of direction, and vertical jumps [2]. In addition to the importance of developing explosive power [3][4][5], aerobic capacity plays a key role, acting as a basis for the recovery of the player [6] and optimal recovery with methods with scientific evidence (foam roll, cold water Immersion and others) after practices and matches play a key role on team sport performance [7].
The scientific literature concludes that sports performance is not limited to the training performed by the athlete, but other factors as well, including both extrinsic and intrinsic variables [8,9]. In this sense, individualized recovery strategies are considered instrumental in achieving and sustaining the highest possible level of performance [10,11], and specifically in elite basketball ecosystems [12], where the players spend a much greater proportion BCI, BIOSIS, CCC, DIIDW, INSPEC, KJD, RSCI and SCIELO, all of high quality], Scopus). This search ended on 31 December 2021. The search terms included a mix of medical subject headings (MeSH) and free-text words for key concepts related to recovery, young, basketball, and players. The following search equation was used to find the relevant articles: ["sleep basketball" (MeSH Terms) OR"sleep" (all fields)] AND ["basketball" (all fields)]. We have also done it in Spanish, using the formula ["sleep basketball " (MeSH TERMS) OR "sleep" (all the fields)] AND ["Basketball (all fields)] OR ["sleep" (all fields)] AND ["Basketball" (all fields)]. There were no filters applied to the athlete's physical fitness level, race, or age to increase the power of the analysis. The search has been carried out independently by two authors, JOL and J.C-G, disagreements were discussed with the third author (S.B) The search terms selected were limited to sleep and basketball. No other terms were used additionally to increase the power of analysis. Through this equation, all relevant articles from this given field were obtained. The reference sections of all identified articles were also examined using the "Snowball Methods" strategy, based on the examination of reference sections of the selected articles [32].
This search method allowed us to apply the PICOS Model [33] on all the results obtained, which was the main inclusion-exclusion criterion of the articles for analysis. These database reviews have been carried out with the data obtained up to December 2021. Regarding the data collection process, the methodology used was the reading of each of the selected articles by the authors of this review. After carefully reviewing all articles, the authors finally excluded 3 more articles (2 opinion articles and one narrative review [18,26,34]). Hence, 25 articles were included in this study. The authors presented a summary table with the most relevant information of each study, in an easy-to-digest format, which in turn, helped the authors analyze, categorize and classify all the data necessary to carry out this review (Table 3). Titles and abstracts were selected to review the full text. Two authors (J.O-L. and J.C-G.) searched for independently published studies, and disagreements on all outcomes were resolved by a third author (S.B)

Inclusion and Exclusion Criteria
Studies included in this systematic review had to meet the following inclusion criteria: (I) the study population comprised basketball players (College level or above); (II) sleep measurements were recorded; (III) the effects of sleep were compared with one or more indicators of basketball performance; and (IV) study designs included all possible designs: quantitative, qualitative, and/or mixed-method model with experimental, quasi-experimental, and/or non-experimental research design, utilizing primary and/or secondary data sources.
Again, in our case, the PICOS model would be as follows: Professional, semi-professional, college, amateur and disabled basketball players (P); Effects of sleep on performance (I); Not applicable (C); Relationship of sleep with specific performance in basketball (O); and all, including quantitative, qualitative, and/or mixed-method model witexperimental, quasi-experimental, and/or non-experimental research design, utilizing primary and/or secondary data sources (S).
All search titles and abstracts were collated to identify duplicates and possible missing studies. Titles and abstracts were screened for further full-text review. The search for published studies was conducted independently by two authors (JOL and h JC-G) and disagreements were resolved through discussions with another author (M.S).
We have also applied the OLE scale [30], as follows (Table 1):

Selection of Studies
Once the inclusion and exclusion criterion has been applied, the information obtained was evaluated by the two authors (JOL and JC-G) to define their selection. The year of publication, the authors, the sample size, the type of study and the characteristics of each of them were taken into account. Afterwards, all the relevant information was put into an Excel sheet (Microsoft Inc, Seattle, WA, USA) independently by two authors (JOL and JC-G) and the disagreements were resolved through discussions with the third author (J.CH).

Extraction of Information
Once the works to be analyzed were selected, the following information was designated from each of them: year, author and publication; sample size; applied methodology; variables analyzed; results.

Assessment of the Quality of the Studies
Considering the potential limitations of the studies included in this systematic review, and in order to draw reliable conclusions, the Cochrane Collaboration Guidelines were followed [27]. Thus, the two authors independently assessed methodological quality and risk of bias (JOL and JC-G), while disagreements were resolved by a third-party assessment (T.H).
In the Cochrane Risk of Bias tool [28], the following elements were included and divided into different domains: (1) selection bias (elements, random sequencing, allocation and concealment), (2) performance bias (blinding of participants and staff), (3) detection bias (blinding of outcome assessment), (4) assertion bias (incomplete outcome data), (5) reporting bias (selective reporting) and (6) other bias (other sources of bias). The assessment of risk of bias was characterized as (a) low-risk (plausible bias that is not likely to seriously alter the results), (b) unclear risk (plausible bias that raises some doubt about the results), or (c) high risk (plausible bias that seriously weakens confidence in the results). Finally, observational studies such as those reviewed in this paper may suffer from this possibility; In addition, there is the possible limitation of the existence of different ways of reacting to sleep in each individual (since it is impossible to know whether the athletes with the best sleep outcomes are performing better on the court or they just happen to be the best players.) However, the results appear robust in terms of their conclusions.
We attach a summary table with all the relevant information of the selected articles (Table 3):

Results
The initial search of scientific literature related to our systematic review presented a result of 50 total articles, of which a total of 25 were excluded from the study, as they were not relevant or related to sleep and its role in basketball (they did not meet the inclusion criteria) (Figure 1).

Results
The initial search of scientific literature related to our systematic review presented a result of 50 total articles, of which a total of 25 were excluded from the study, as they were not relevant or related to sleep and its role in basketball (they did not meet the inclusion criteria) (Figure 1).

Main Search
The variables analyzed in these studies focused on the comparison of different sleep interventions (such as sleep quality and extension, sleep hygiene, the use of electronic devices during the night ...) with the performance of athletes, measured with different magnitudes of basketball, all of them typical of this sport. This type of relationship has been found in 8 articles [35,38,45,48,49,54,56,58]. We have also seen an important emphasis in the study of the relationship of workloads and the quality and extent of sleep of athletes (6 articles were dedicated to this topic [37,39,46,47,50,57]), as well as in the relationship of

Main Search
The variables analyzed in these studies focused on the comparison of different sleep interventions (such as sleep quality and extension, sleep hygiene, the use of electronic devices during the night . . . ) with the performance of athletes, measured with different magnitudes of basketball, all of them typical of this sport. This type of relationship has been found in 8 articles [35,38,45,48,49,54,56,58]. We have also seen an important emphasis in the study of the relationship of workloads and the quality and extent of sleep of athletes (6 articles were dedicated to this topic [37,39,46,47,50,57]), as well as in the relationship of travel, jet-lag and traveler fatigue and its effect on the sleep of the athlete (9 articles analyzed these topics). Also related to circadian rhythms and their association with basketball performance, a study analyzed the best times of the day to train based on these rhythms, comparing the consequences of training in the morning with evening workouts [43]. On the other hand, one study has linked sleep and workloads to the possibility of injury of the athlete [52] and two works have related nutrition and ergogenic aids with the subsequent quality and extension of sleep [53,59].

Assessment of the Quality of the Studies
As for the participants of the studies, we divide them among empirical studies with athletes of different characteristics (professionals vs. amateur) and reviews of studies or complete seasons of a specific professional competition. All these studies provide us with relevant information, so they have been included in our review. Regarding the professional competitions analyzed, 4 reviewed articles contain 99 studies on the subject and a total of 14,404 NBA games analyzed [18,44,54,60]. Furthermore, the newest study from Charest et al [61]  Therefore, there is a clear bias in the studies towards the inclusion of a majority of male athletes. We located one study [40] that precisely details the difference in sleep between male and female athletes. However, this study was discarded because of the use of the REST-Q scale as a sleep measure [55].
As for the chronology of the publications studied (Figure 2), it should be noted that of these 22 most recent articles, 15 [43,45,62] and of previous years, we have examined an article of 1997 [60], another of 2011 [58] and another of 2012 [42]. We see therefore that researchers in sports sciences are taking the study of sleep very seriously, which proves to be one of the keys in the health and performance of athletes [35,63].
travel, jet-lag and traveler fatigue and its effect on the sleep of the athlete (9 articles analyzed these topics). Also related to circadian rhythms and their association with basketball performance, a study analyzed the best times of the day to train based on these rhythms, comparing the consequences of training in the morning with evening workouts [43]. On the other hand, one study has linked sleep and workloads to the possibility of injury of the athlete [52] and two works have related nutrition and ergogenic aids with the subsequent quality and extension of sleep [53,59].

Assessment of the Quality of the Studies
As for the participants of the studies, we divide them among empirical studies with athletes of different characteristics (professionals vs. amateur) and reviews of studies or complete seasons of a specific professional competition. All these studies provide us with relevant information, so they have been included in our review. Regarding the professional competitions analyzed, 4 reviewed articles contain 99 studies on the subject and a total of 14,404 NBA games analyzed [18,44,54,60]. Furthermore, the newest study from Charest et al [61]  Therefore, there is a clear bias in the studies towards the inclusion of a majority of male athletes. We located one study [40] that precisely details the difference in sleep between male and female athletes. However, this study was discarded because of the use of the REST-Q scale as a sleep measure [55].

Risk of Bias
Risk of bias are the likelihood that features of any study may give misleading results. As such, we analyzed the following: in relation to the risk of selection bias (Table 4), 18 studies have been categorized as low risk [18,26,38,39,[42][43][44]47,[51][52][53][54][56][57][58][60][61][62], 8 as medium risk [22,35,37,45,[48][49][50]59] and two high-risk [18,46]. Regarding the risk of realization bias, one study was classified as high risk [18], 6 studies as medium risk [37,45,46,48,49,59], while all other studies were categorized as low risk. One study has been categorized as high risk of detection bias [18], 8 medium risks [37,45,46,[48][49][50]54,59], with the rest of the studies classified as low risk. If we talk about the risk of assertion bias, two studies [18,46] have been classified as high risk, 6 intermediate risk studies [37,45,46,49,54,59] and the rest have been considered as low risk. In addition, in reference to reporting risk bias, two studies are at high risk [18,46] and 4 medium risk [50,54,59,60] and the rest are considered to be low risk. And finally, in reference to other risks of bias, only two studies [18,46] have been considered high risk and 7 others medium risk [35,38,42,43,51,54,59] (Figure 3).  All the information analyzed from these 25 articles reveals that the conclusions reached by the different authors after their respective analyses are significantly homogeneous. We could say that, in general, studies present a clear relationship between sleep and sports performance, in this case in basketball. There are different topics addressed in relation to sleep and performance, which we could classify as follows: • First, 8 studies concluded that the relationship between performance and the quantity and quality of sleep is directly proportional, and that, therefore, the greater the hours of sleep and the greater the continuity of sleep across a sleep period, the subsequent performance on the basketball court increases [22,35,36,48,49,54,56,58].
Two of them also reveal that among young (university) players, men recover better than women [40] and, in addition, that although young players are aware that sleep is key in recovery, only 24% of them consider it as such [22].

•
Besides, 6 studies focused on training or match loads applied to players, being four of them related to high stress loads with worse nights' sleep [37,39,47,57], while only one study did not find this relationship [50]. Another study [46] found no relationship between sleep quality and associated Rate of Perceived Exertion (RPE) of subsequent training (that is, it is concluded that high training loads affect the quality of sleep on the night following the training session, so performance the next day will be negatively affected). This should be taken into account when planning recovery for these tougher sessions, where it would be convenient to lengthen the number of hours slept by players.

•
Only one study examined the likelihood of injury compared to the number of hours slept, concluding that more sleep equates to reducing future chances of injury [52].

•
Six articles studied the relationship between travel and circadian rhythms, concluding that traveling, due to fatigue, jet lag, time changes etc. significantly negatively affect the sleep, recovery and performance of players [18,26,44,51,60,61]. One of the studies highlights that the relationship of performance and sleep is affected differently to each individual [62], so each coaching staff should know this individual variability for their players. On the other hand, another study [43] highlights how circadian rhythms affect daily energy peaks, underpinning the fact that you train and compete better in the afternoon than in the morning. This information is also closely related to the calendars of professional teams, being the NBA and their trips to different time zones its paradigmatic case. This should be taken into account in the  All the information analyzed from these 25 articles reveals that the conclusions reached by the different authors after their respective analyses are significantly homogeneous. We could say that, in general, studies present a clear relationship between sleep and sports performance, in this case in basketball. There are different topics addressed in relation to sleep and performance, which we could classify as follows: • First, 8 studies concluded that the relationship between performance and the quantity and quality of sleep is directly proportional, and that, therefore, the greater the hours of sleep and the greater the continuity of sleep across a sleep period, the subsequent performance on the basketball court increases [22,35,36,48,49,54,56,58].
Two of them also reveal that among young (university) players, men recover better than women [40] and, in addition, that although young players are aware that sleep is key in recovery, only 24% of them consider it as such [22].

•
Besides, 6 studies focused on training or match loads applied to players, being four of them related to high stress loads with worse nights' sleep [37,39,47,57], while only one study did not find this relationship [50]. Another study [46] found no relationship between sleep quality and associated Rate of Perceived Exertion (RPE) of subsequent training (that is, it is concluded that high training loads affect the quality of sleep on the night following the training session, so performance the next day will be negatively affected). This should be taken into account when planning recovery for these tougher sessions, where it would be convenient to lengthen the number of hours slept by players.

•
Only one study examined the likelihood of injury compared to the number of hours slept, concluding that more sleep equates to reducing future chances of injury [52]. • Six articles studied the relationship between travel and circadian rhythms, concluding that traveling, due to fatigue, jet lag, time changes etc. significantly negatively affect the sleep, recovery and performance of players [18,26,44,51,60,61]. One of the studies highlights that the relationship of performance and sleep is affected differently to each individual [62], so each coaching staff should know this individual variability for their players. On the other hand, another study [43] highlights how circadian rhythms affect daily energy peaks, underpinning the fact that you train and compete better in the afternoon than in the morning. This information is also closely related to the calendars of professional teams, being the NBA and their trips to different time zones its paradigmatic case. This should be taken into account in the future to improve the quality of life of the players themselves, as well as their performance and their risk of injury.
In 3 of these articles, the importance of the competitive calendar was also highlighted, in a similar way, and how it negatively affects sleep, recovery and performance [18,26,51] due to its corresponding saturation of matches and the short time to recover between them, especially when playing "back-to-back" matches.

•
3 articles linked sleep quality to vigor and mood in athletes with disabilities (wheelchair basketball players) [38,45] or isolated by Covid19 [36].

•
In two articles that related sleep to nutrition and ergogenic aids, it was concluded that caffeine does not affect basketball performance [53] and that the food that the athlete eats for an entire day affects sleep more than the glycemic index of his last meal, dinner [59].

•
One article concludes that red light therapies before sleep affect athletes in a positive manner, allowing them a better quality of sleep and a better recovery after training or match [42]. This type of therapy could be very useful to improve the recovery of athletes in very saturated periods of competition or when the competitive calendar forces them to travel continuously.
After having analyzed the 25 publications mentioned, we found that despite the existing attempts to modernize and improve basketball analysis [64,65], scientific information about sleep and its practices does not seem to have yet reached the offices of clubs or even professional basketball competition organizations. We believe that the practical applications derived from the conclusions of this review should be taken into account by clubs and leagues, and that this could have very positive consequences on the health of the players and their performance, and therefore, on quality and spectacle and even on long-term economic viability.
All the above information is summarized in the following Table 5:

Discussion
The present systematic review has focused on reviewing the scientific work that has been published regarding the effect that sleep can have on the performance of basketball players. After analyzing 25 articles, the majority of them published in recent years, we can conclude that sleep does affect basketball performance and its related injuries. In addition, the results show that there are several factors, from trips to external and internal loads or training schedules, that negatively affect the rest of the athlete and that are not being considered when applying recovery strategies.

Sleep and Performance
The main conclusion of this systematic review is that sleep does have a clear impact on performance in the sport of basketball [34], affecting not only the statistics of the players [48,54,56,58] but also to the chances of injury [52]. Therefore, it seems undeniable that sleep should be a fundamental control variable by the coaching staff of any team [66].

Sleep and Workloads
The results of the studies analyzed show that the loads of the workouts affect sleep in the following way: higher loads prevent athletes from having quality sleep that same night [36]. This affects performance the next day [39,47,57], so coaches should take this into account when planning and deciding their workout intensity and schedules.

Sleep and Circadian Rhythm
Circadian rhythms affect all players and coaches and they determine the best and worst times of the day to train and compete [43]. This is not being taken into account when scheduling training or competitions [62]. If this variable were taken into account consistently, the performance of players could be increased in a similar way. Future lines of research should focus on this to further develop this idea.

Sleep and Traveling
In professional competitions, it is increasingly common to travel hundreds or thousands of kilometers per week in order to meet the assigned matches [26,61]. This has a significant toll players health and performance: long journeys negatively affect recovery of athletes, through factors such as the physical toil of travel and disruptions to the circadian rhythm [18,51,60]. Therefore, the performance of the players tends to be worse and the probability of injuries, higher. Again, this is not being taken into account when choosing competition calendars. The respective organizations should take this into account to improve competitions and better protect players, who are the most important assets that clubs have [18].

Conclusions
This work of review of the most current scientific literature shows that sleep and circadian rhythms are very fashionable topics that are acquiring a lot of importance in the competitive environment of basketball in recent seasons. There are finally solid scientific conclusions that crown sleep as a basic practice of recovery of the athlete and it is clear that it affects their performance and health. Dismissing the use of good sleep practices and not having into account the diverse circadian rhythms of our athletes can have deleterious consequences on basketball performance.

Practical Applications
Education in sleep hygiene and its important consequences should come from the hand of the staff of each team, to enhance the only natural (and free) tool that exists to recover the body from the stress to which we subject it in each training and game [67,68]. Players must understand how to create a positive pre-sleep environment (avoiding the use of stimulants, social media and bright light before going to bed, especially blue light, using amber tinted glasses and night-shift-mode for the electric devices as a countermeasure, setting a cooler and quiet environment without distractions) and how this can affect their performance, both in numbers and statistics and in injuries.
On the other hand, coaches should take more into account natural circadian rhythms when scheduling loads for both morning and evening workouts, allowing more hours of sleep for their players after a particularly hard training day or match. The same applies after a long and heavy trip, when facing the next training. Sleep should be considered as one more recovery practice and therefore should have the same importance as the usual practices in these cases (cold immersion, ergogenic aids, massages, compression garments . . . ).
As for professional basketball clubs and leagues, it seems necessary that more attention be paid to the negative impact of travel on the performance and health of players. Now that it is clearer how they suffer in their performance and how the chances of injury increase, both factors that condition the spectacle of this sport and thus, are key when it comes to attracting fans (and therefore, the money necessary to keep everything moving), sleep cannot be considered trivial. Moreover, in the main professional leagues of the world (NBA, Euro league), where long trips to different time zones are common, emphasis should be placed on providing enough hours of rest between trips, on avoiding the classic schedule of two consecutive games (back-to-back games) and even the time of the game itself. We must remember that in the NBA it has been shown [44] how when a team from the West travels to the East the time of the game benefits him by coinciding with its peak of the circadian rhythm. We believe that all these recommendations can increase the value of the professional sport of basketball at several levels, from the purely sports' side to the organizational and even throughout the economic part. By improving both the show and the organization of it, it must necessarily result in a greater attraction on the part of the spectator and a more efficient use of budgets by the clubs.
All the information collected in this work should be taken into account by both the players and the coaches of the teams (staff members are also affected by these conditions, as Calleja-González et al pointed out in 2021 [66]), the managers of the clubs and by the organizations of the professional leagues themselves, in order to facilitate a safer and more coherent environment with the circadian rhythms and recovery times of the human being.

Future Lines of Research
Basketball is a sport that requires more accurate future research on sleep and with a larger population and duration over time. We know that at the elite of the sport, the NBA [26], attention is beginning to be drawn to such an important issue [18] and we believe that progress has to be made in that direction. The approach to sleep, therefore, should be personalized, always taking into account the individuality of each player and being able to provide a tailor-made solution.

Limitations
The number of articles related to this topic that have been based on professional players is very small, due to the difficulty of examining this type of profile. That is why in this review it has been decided to also include the information provided by the studies with semi-professional and amateur players.
Regarding the risk of bias and the quality of the methodology used, disagreements have been discussed between the two authors, JOL and JC-G, until reaching an agreement on the subject.