**2. Physical activity and academic achievement**

attention on implementing physical activity interventions (e.g., increasing the number of physical education hours per week) with the aim to improve the pupils' health as well as to reduce the obesity levels among adolescents. Nevertheless, the physical education is considered a second layer in the educational process due to the concern of educative policies about reaching the adequate standards of academic achievements among students. Then, the educative policies establish the reduction and, in some cases, the elimination of physical education classes. The traditional education research considers that pupils should employ the time studying the theoretical subjects instead on spending time doing physical activities [5–9].

However, the recent research supports that physical activity practice is related with improvements in cognitive and memory functions of the brain [10]. In fact, the increase of physical activity may improve the school performance and academic achievements in children and adolescents [11]. For instance, it seems to exist a positive relationship between motor competence (achieved through physical activity practice) and academic performance [12]. Specifically, in the female students, the academic achievement was associated with the practice of vigorous physical activity, whereas in male students the academic achievement was associated with a greater extent with fitness [13]. Overall, the evidence supports that the practice of physical activity leads to improvements on both cognitive and academic

On the other hand, regular sleep is fundamental on health and growth of children and adolescents. Thus, the lack of sleep and sleep deprivation is associated with adverse physical and psychological consequences. The scientific research indicates that a good sleep quality is not only fundamental in determining a good health state, but it is also a main component for a good quality of life and academic performance [16]. For instance, it has been found that children and teenagers who suffer from circadian misalignment (i.e., insufficient sleep during the weekdays and oversleeping on weekends) have a lower academic, cognitive and sport

There are a lot of researches that have showed the strong links between health behaviors, psychosocial behaviors and academic achievement [17]. International organizations, such as the World Health Organization, in their different proposals have stressed the need for schools to be the core of health promotion, which should be translated into curricular improvements proposed to develop healthy habits, and the involvement of families and social agents to support these initiatives. The objective is the development of skills and competences that favor

At present, studies that have analyzed the relationships between healthy behaviors and academic performance have increased markedly. However, what is referenced when talking about health and healthy behaviors? Health is a state of complete emotional and physical well-being. This concept during the school age has been focused on those who favor good nutrition; do not use tobacco, alcohol or drugs; do not show violent behaviors; practice safe

performance than those who rest more regularly and with better sleep quality [3].

the health of students and a better adaptation to life [18].

sex and do not practice physical activity [19].

performance [14, 15].

214 Health and Academic Achievement

**1.1. Health and education**

During the past 10 years, the effect of physical activity on academic achievement has become a more important topic with a growing number of scientific works published about this issue [30, 31]. Throughout this section, we (i) discuss the influence of physical activity upon academic achievement; (ii) analyze which psychophysiological effects of physical activity support that association and (iii) raise some recommendations of physical practice when aiming to improve the academic achievement. The academic performance can reflect several factors influencing the school success. For instance, Rasberry et al. [32] employed three categories: (a) cognitive skills and attitudes, (b) academic behaviors and (c) academic achievement. In this section, we focus our attention on the academic achievement defined as the test scores in subjects such as mathematics, reading, language art, social sciences and any other formal assessment regarding school areas or subjects.

#### **2.1. More studying and less doing sports?**

At a first glance, parents are concerned about the academic achievement of their children. Then, they may think that the involvement in physical activities could hamper the forthcoming academic achievement, given that physical activity practice involves a great deal of effort and time. Hence, the time that boys and girls spend practicing physical activity results in a reduction of time dedicated to academic activities (i.e., studying, doing homework, reading, etc.). However, this does not seem to be applied to extracurricular activities like sports or extracurricular physical education. The scientific research has provided consistent evidence that time away from the sedentary study in favor of physical activities does not decrease the academic achievement [33, 34]. Some studies have proved that an increase for the amount of time dedicated towards physical activities (such as physical education or sport-related activities) does not impair the academic performance [35]. Furthermore, even when redistributing curriculum time in favor of aerobic physical activities, far from hinder, it improves the academic achievement of children [36]. Regarding the effort-demanding argument, one might think that the involvement in sports trainings could also lead to fatigue among children and adolescents. In fact, sedentary time (contrary to sports-related activities) is related to lower scores in academic tests (reading fluency, reading comprehension, arithmetic) [37]. Indeed, time spending for watching TV, playing videogames or interacting with mobile devices and social networks hinder the academic achievement [32]. In sum, scientific literature points towards a positive association between physical activity involvement and academic achievement among students from 4 to 18 years.

seems to be directly related to academic achievement. For instance, poorer motor performance (i.e., shuttle run, balance and box and block tests) was associated with worse academic reading and arithmetic skills in children (6–8 years). More recently, Ruiz et al. [44] explored the relationships between academic achievements (expressed as the final mark of the participants across the academic year, in a 10-based scale) and the motor coordination (Test SportComp of motor coordination [45]) among youth students (11–16 years old). Their results showed weak, but consistent, positive relationships between 4 of 5 motor coordination tests and academic

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Other studies have been focused on the effect of intervention in physical activity on academic achievement and/or that relationship across the time. For instance, Bezold et al. [46] published a vast longitudinal study with 83,111 children enrolled in grades 6–8 in the first year of the study. For 5 years, fitness scores (determined by three tests: aerobic capacity, pushup and curl-up tests) and academic performance (measured following Mathematics and English Language Arts test scores) were monitored. Both boys and girls who experienced a substantial increase in fitness resulted in an improvement in academic ranking by 0.36–0.38 percentiles compared with the reference group. In addition, in boys and girls, a substantial decrease in fitness was associated with significant decline in academic performance compared with the reference group. Then, pupils who increase their fitness also improved their academic achievement, whereas pupils who decreased their fitness worsened their scores. In another longitudinal study [47], the physical activity of fifth and seventh grade children was assessed (i.e., aerobic capacity, upper body strength and endurance, body composition, abdominal strength and endurance, flexibility and trunk extensor strength and flexibility) to detect children in the healthy fitness zone or children in the needs improvement zone. After 2 years, pupils who stayed in the healthy fitness zone had higher scores (in all academic domains: mathematics, reading, science and social studies) than those who remained in the needs improvement zone. The children who moved between healthy fitness zone and needs improvement zone (in any direction) obtained in between aca-

demic scores, indicating a longitudinal effect of fitness on academic achievement.

Donnelly et al. [48] carried out an intervention of 90 minutes of moderate-to-vigorous physical activity for 2 years with children from grades second and third. Children who received the intervention experimented improvements in all the academic domains: reading, composite, mathematics and spelling scores across the 3 years' baseline with respect to control schools. Other studies have investigated the effect of increasing physical activity among school-aged children. Despite some contradictory results [27, 49], the systematic review of intervention studies supports that physical activity is positively associate with the academic achievement [50].

Stronger evidence of the benefits of physical activity on academic achievement arises from systematic reviews and meta-analyses [31, 32, 51–57]. In this sense, one of the first meta-analysis about cognitive functions reported a 0.25 of average effect size of physical activity cognitive functions [51]. Another meta-analysis [50] found that chronic physical activity interventions

performances.

*2.2.2. Longitudinal and intervention studies*

*2.2.3. Meta-analysis and systematic reviews*

#### **2.2. Physical activity and academic performance**

#### *2.2.1. Cross-sectional studies*

A first group of cross-sectional studies informs about relationships of physical activity habits with the academic scores [38]. In this sense, self-reported time spent on physical activity is directly related (and screen time inversely) to higher scores among children aged between 11 and 18 years [39, 40]. Another research employing objectively measured tests has also found positive relationships between physical activity and academic achievement. For instance, the work by Dwyer et al. [41], which involved 7961 participants from 7 to 15 years, informed how academic achievement (5-point scale) was positively related (in all ages and in both sex) with cardiorespiratory endurance (i.e., 1.6 km run), muscular force (e.g., sit-ups) and time of physical activity. In another study where physical activity was monitored, lower academic performance (all domains: language, reading, spelling, writing and numeracy) was strongly related to higher levels of sedentary time among children (9–11 years old). In contrast, moderate-to-vigorous physical activity was related with writing and numeracy scores. Haapala et al. [42] reported that pupil between 6 and 8 years showed positive relationships between levels of physical activities (measured with accelerometers) and reading and arithmetic skills. Interestingly, the combination of lower levels of moderate-to-vigorous physical activity with high sedentary time was related to poorer reading skills (fluency and comprehension) in boys.

In another similar study, clear associations between moderate-to-vigorous physical activity and academic performance were not found among 10-year-old children [43]. In addition, academic achievement was positively correlated to both aerobic fitness (i.e., intermittent running field test) and motor skills measured with a battery of three test (i.e., catching with one hand, throwing at a wall target and shuttle run test). Along these lines, motor skill performance seems to be directly related to academic achievement. For instance, poorer motor performance (i.e., shuttle run, balance and box and block tests) was associated with worse academic reading and arithmetic skills in children (6–8 years). More recently, Ruiz et al. [44] explored the relationships between academic achievements (expressed as the final mark of the participants across the academic year, in a 10-based scale) and the motor coordination (Test SportComp of motor coordination [45]) among youth students (11–16 years old). Their results showed weak, but consistent, positive relationships between 4 of 5 motor coordination tests and academic performances.

#### *2.2.2. Longitudinal and intervention studies*

academic achievement, given that physical activity practice involves a great deal of effort and time. Hence, the time that boys and girls spend practicing physical activity results in a reduction of time dedicated to academic activities (i.e., studying, doing homework, reading, etc.). However, this does not seem to be applied to extracurricular activities like sports or extracurricular physical education. The scientific research has provided consistent evidence that time away from the sedentary study in favor of physical activities does not decrease the academic achievement [33, 34]. Some studies have proved that an increase for the amount of time dedicated towards physical activities (such as physical education or sport-related activities) does not impair the academic performance [35]. Furthermore, even when redistributing curriculum time in favor of aerobic physical activities, far from hinder, it improves the academic achievement of children [36]. Regarding the effort-demanding argument, one might think that the involvement in sports trainings could also lead to fatigue among children and adolescents. In fact, sedentary time (contrary to sports-related activities) is related to lower scores in academic tests (reading fluency, reading comprehension, arithmetic) [37]. Indeed, time spending for watching TV, playing videogames or interacting with mobile devices and social networks hinder the academic achievement [32]. In sum, scientific literature points towards a positive association between physical activity involvement and academic achievement among students

A first group of cross-sectional studies informs about relationships of physical activity habits with the academic scores [38]. In this sense, self-reported time spent on physical activity is directly related (and screen time inversely) to higher scores among children aged between 11 and 18 years [39, 40]. Another research employing objectively measured tests has also found positive relationships between physical activity and academic achievement. For instance, the work by Dwyer et al. [41], which involved 7961 participants from 7 to 15 years, informed how academic achievement (5-point scale) was positively related (in all ages and in both sex) with cardiorespiratory endurance (i.e., 1.6 km run), muscular force (e.g., sit-ups) and time of physical activity. In another study where physical activity was monitored, lower academic performance (all domains: language, reading, spelling, writing and numeracy) was strongly related to higher levels of sedentary time among children (9–11 years old). In contrast, moderate-to-vigorous physical activity was related with writing and numeracy scores. Haapala et al. [42] reported that pupil between 6 and 8 years showed positive relationships between levels of physical activities (measured with accelerometers) and reading and arithmetic skills. Interestingly, the combination of lower levels of moderate-to-vigorous physical activity with high sedentary time was related to poorer reading skills (fluency and comprehension) in boys. In another similar study, clear associations between moderate-to-vigorous physical activity and academic performance were not found among 10-year-old children [43]. In addition, academic achievement was positively correlated to both aerobic fitness (i.e., intermittent running field test) and motor skills measured with a battery of three test (i.e., catching with one hand, throwing at a wall target and shuttle run test). Along these lines, motor skill performance

from 4 to 18 years.

216 Health and Academic Achievement

*2.2.1. Cross-sectional studies*

**2.2. Physical activity and academic performance**

Other studies have been focused on the effect of intervention in physical activity on academic achievement and/or that relationship across the time. For instance, Bezold et al. [46] published a vast longitudinal study with 83,111 children enrolled in grades 6–8 in the first year of the study. For 5 years, fitness scores (determined by three tests: aerobic capacity, pushup and curl-up tests) and academic performance (measured following Mathematics and English Language Arts test scores) were monitored. Both boys and girls who experienced a substantial increase in fitness resulted in an improvement in academic ranking by 0.36–0.38 percentiles compared with the reference group. In addition, in boys and girls, a substantial decrease in fitness was associated with significant decline in academic performance compared with the reference group. Then, pupils who increase their fitness also improved their academic achievement, whereas pupils who decreased their fitness worsened their scores. In another longitudinal study [47], the physical activity of fifth and seventh grade children was assessed (i.e., aerobic capacity, upper body strength and endurance, body composition, abdominal strength and endurance, flexibility and trunk extensor strength and flexibility) to detect children in the healthy fitness zone or children in the needs improvement zone. After 2 years, pupils who stayed in the healthy fitness zone had higher scores (in all academic domains: mathematics, reading, science and social studies) than those who remained in the needs improvement zone. The children who moved between healthy fitness zone and needs improvement zone (in any direction) obtained in between academic scores, indicating a longitudinal effect of fitness on academic achievement.

Donnelly et al. [48] carried out an intervention of 90 minutes of moderate-to-vigorous physical activity for 2 years with children from grades second and third. Children who received the intervention experimented improvements in all the academic domains: reading, composite, mathematics and spelling scores across the 3 years' baseline with respect to control schools. Other studies have investigated the effect of increasing physical activity among school-aged children. Despite some contradictory results [27, 49], the systematic review of intervention studies supports that physical activity is positively associate with the academic achievement [50].

#### *2.2.3. Meta-analysis and systematic reviews*

Stronger evidence of the benefits of physical activity on academic achievement arises from systematic reviews and meta-analyses [31, 32, 51–57]. In this sense, one of the first meta-analysis about cognitive functions reported a 0.25 of average effect size of physical activity cognitive functions [51]. Another meta-analysis [50] found that chronic physical activity interventions had a significant small-to-moderate effect on cognition (effect size of 0.46) in youth (school age). Also, positive effects of physical activity interventions on cognition were identified when compared to no treatment (0.80). Other study [51], focused on secondary school (13–18 years old), revealed that most of the studies reviewed showed positive associations between moderate-to-vigorous physical activity and academic performance. Likewise, it has been reported that physical activity (all the types analyzed: resistance/circuit training, physical education programs, aerobic training), and regardless the methodology of study, has a significant effect (average of effect size of 0.32) on cognitive functions (measured as perceptual skills, academic readiness, achievement, math and verbal test) of school-aged children (aged 4–18 years).

Several mechanisms underpin the effects of physical activity on cognitive functions during childhood and young adulthood: increased synaptic plasticity, cerebral circulation (blood volume), hippocampal neurogenesis, increases in neurochemicals (e.g., norepinephrine and dopamine), greater white matter integrity, upregulation of growth factors, efficiency of the prefrontal and parietal cortices and structural changes in the hippocampus and cerebellum [60–62]. These findings suggest that physical activity influences baseline electrocortical function and, thus, it might affect cognitive operations. In fact, a growing body of literature, including fMRI (functional magnetic resonance imaging) and ERP (event-related potential) studies, indicates that physical activity interventions produce improvements in brain function accompanied by improvements in

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For example, research indicates that when practicing moderate-to-vigorous aerobic exercises, prefrontal cortex and parietal/posterior cingulate cortex are activated to sustain the activity. These are relevant neural networks involved in complex cognitive functions, such as reading comprehension or mathematical calculations, which may support the positive relationships between both activities (i.e., physical and cognitive) [57]. More recently, neurophysiologicalbased studies pointed towards new connections between cognitive and motor functions, as the relationships of activation between prefrontal cortex and collateral cerebellum. This latter area is believed to play an important role not only in control and adaptation of movements but also in cognitive functions. In other words, improving the functioning of certain neural

The review done by Chu et al. [63] highlighted positive associations between cardiorespiratory fitness and the academic achievement (total academic scores). Cardiorespiratory fitness has been consistently reported to exhibit a positive association with mathematics, social and science studies, spelling and language arts scores in both children and adolescents. Specifically, up to date, the available research is scarce when studying the relationships between cardiorespiratory adaptations and the academic achievement. For instance, cardiovascular performance (measured as maximal workload during cycloergometer exercise) does not seem to be associated with reading or arithmetic skills [64]. Another study supported the lack of relationship between cardiovascular capacity (heart rate) and the academic performance [65]. That is, cardiorespiratory fitness seems to improve the academic achievement by means of neurophysiological mediating effects rather than cardiorespiratory adaptations. For instance, children with higher cardiorespiratory fitness exhibited larger event-related brain potentials (ERBp), which are involved in neurocognitive process like cognitive operations [60]. Other associations among fitness measures like muscular strength, muscular endurance or flexibility with the academic achievement are unclear. They seem to have a positive relationship, but remains weak or unrelated when adjusted for potential mediators (i.e., socio-demographic

Finally, the body of research suggests that other types of low-intensity trainings like practice motor skills may improve the cognitive performance. For instance, coordination training in adults (aimed to improve the efficiency of complex body movements) was associated with

networks through physical exercise may benefit the academic performance.

executive functions [62].

*2.3.2. Cardiovascular fitness*

factors, body mass index (BMI), etc.) [57, 60, 66].

A common concern expressed in the systematic reviews is the low methodological quality of part of the published works [41, 58]. Yet, when the risk of bias of existing studies is neatly assessed, positive associations between physical activity and academic achievement are found in all three cross-sectional, randomly controlled trials and longitudinal studies [47, 59]. In sum, a positive association between physical exercise programs and academic performance has been reported consistently by recent systematic reviews and meta-analyses [53].

#### **2.3. Underlying processes of the effect of physical activity on academic achievement**

As was previously seen, the available research has previously confirmed the plausible relationships between physical activity and the academic achievement. The next question to address would be what are the mediating factors among them? In other words, where does the effect of physical activity on academic achievement come from? To respond these questions, we consider the hypothetical model of factors proposed by Donnelly et al. [38] (**Figure 1**).

#### *2.3.1. Cognitive functions*

As Erickson et al. [60] claimed, higher fit and more active preadolescent children show more efficient patterns of brain activity and superior cognitive performance and scholastic achievement.

**Figure 1.** Hypothetical model of factors associated with improved academic achievement (from Donnelly and Lambourne [5]).

Several mechanisms underpin the effects of physical activity on cognitive functions during childhood and young adulthood: increased synaptic plasticity, cerebral circulation (blood volume), hippocampal neurogenesis, increases in neurochemicals (e.g., norepinephrine and dopamine), greater white matter integrity, upregulation of growth factors, efficiency of the prefrontal and parietal cortices and structural changes in the hippocampus and cerebellum [60–62]. These findings suggest that physical activity influences baseline electrocortical function and, thus, it might affect cognitive operations. In fact, a growing body of literature, including fMRI (functional magnetic resonance imaging) and ERP (event-related potential) studies, indicates that physical activity interventions produce improvements in brain function accompanied by improvements in executive functions [62].

For example, research indicates that when practicing moderate-to-vigorous aerobic exercises, prefrontal cortex and parietal/posterior cingulate cortex are activated to sustain the activity. These are relevant neural networks involved in complex cognitive functions, such as reading comprehension or mathematical calculations, which may support the positive relationships between both activities (i.e., physical and cognitive) [57]. More recently, neurophysiologicalbased studies pointed towards new connections between cognitive and motor functions, as the relationships of activation between prefrontal cortex and collateral cerebellum. This latter area is believed to play an important role not only in control and adaptation of movements but also in cognitive functions. In other words, improving the functioning of certain neural networks through physical exercise may benefit the academic performance.

#### *2.3.2. Cardiovascular fitness*

had a significant small-to-moderate effect on cognition (effect size of 0.46) in youth (school age). Also, positive effects of physical activity interventions on cognition were identified when compared to no treatment (0.80). Other study [51], focused on secondary school (13–18 years old), revealed that most of the studies reviewed showed positive associations between moderate-to-vigorous physical activity and academic performance. Likewise, it has been reported that physical activity (all the types analyzed: resistance/circuit training, physical education programs, aerobic training), and regardless the methodology of study, has a significant effect (average of effect size of 0.32) on cognitive functions (measured as perceptual skills, academic readiness, achievement, math and verbal test) of school-aged children (aged 4–18 years).

A common concern expressed in the systematic reviews is the low methodological quality of part of the published works [41, 58]. Yet, when the risk of bias of existing studies is neatly assessed, positive associations between physical activity and academic achievement are found in all three cross-sectional, randomly controlled trials and longitudinal studies [47, 59]. In sum, a positive association between physical exercise programs and academic performance

has been reported consistently by recent systematic reviews and meta-analyses [53].

**2.3. Underlying processes of the effect of physical activity on academic achievement**

consider the hypothetical model of factors proposed by Donnelly et al. [38] (**Figure 1**).

*2.3.1. Cognitive functions*

218 Health and Academic Achievement

Lambourne [5]).

As was previously seen, the available research has previously confirmed the plausible relationships between physical activity and the academic achievement. The next question to address would be what are the mediating factors among them? In other words, where does the effect of physical activity on academic achievement come from? To respond these questions, we

As Erickson et al. [60] claimed, higher fit and more active preadolescent children show more efficient patterns of brain activity and superior cognitive performance and scholastic achievement.

**Figure 1.** Hypothetical model of factors associated with improved academic achievement (from Donnelly and

The review done by Chu et al. [63] highlighted positive associations between cardiorespiratory fitness and the academic achievement (total academic scores). Cardiorespiratory fitness has been consistently reported to exhibit a positive association with mathematics, social and science studies, spelling and language arts scores in both children and adolescents. Specifically, up to date, the available research is scarce when studying the relationships between cardiorespiratory adaptations and the academic achievement. For instance, cardiovascular performance (measured as maximal workload during cycloergometer exercise) does not seem to be associated with reading or arithmetic skills [64]. Another study supported the lack of relationship between cardiovascular capacity (heart rate) and the academic performance [65]. That is, cardiorespiratory fitness seems to improve the academic achievement by means of neurophysiological mediating effects rather than cardiorespiratory adaptations. For instance, children with higher cardiorespiratory fitness exhibited larger event-related brain potentials (ERBp), which are involved in neurocognitive process like cognitive operations [60]. Other associations among fitness measures like muscular strength, muscular endurance or flexibility with the academic achievement are unclear. They seem to have a positive relationship, but remains weak or unrelated when adjusted for potential mediators (i.e., socio-demographic factors, body mass index (BMI), etc.) [57, 60, 66].

Finally, the body of research suggests that other types of low-intensity trainings like practice motor skills may improve the cognitive performance. For instance, coordination training in adults (aimed to improve the efficiency of complex body movements) was associated with better attentional control in visual search tasks [63]. The authors argued that those effects can be mediated by changes in brain functioning (i.e., frontal and parietal cortex), although research among children has not been conducted so far. In conclusion, positive relationships and associations between executive functions and physical activity have been found in terms of: (a) moderate-to-vigorous physical activity, (b) aerobic fitness and (c) motor skills [62].

functions affecting the good state of human memory, learning/memory capacity, the brain's optimal function or the neurobehavioral function [78–81]. Therefore, sleep habits are one of the most important daily routines that must be correctly done [82]. The absence, loss or deprivation of sleep has a direct effect on health problems [83–85]. However, when these disorders or inadequate sleep habits occur during the childhood or the adolescence, the academic performance and the physical and physiological development are highly compromised [86–88]. Nowadays, sleep loss is one of the problems that the people should face daily. Usually, it is preferable to sacrifice some of the time dedicated to sleep hoping that this time will not have a negative impact during our daily activities such as studying, working, etc. [83]. However, this behavior has a direct and negative effect on physical (fatigue), neurocognitive (impairment in learning and

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memory) and psychomotor performances (impairment in psychomotor activities) [89].

Specifically, the importance of sleep loss increases during the childhood and the adolescence due to the important role that plays in learning capacity, school performance and memory consolidation [72, 73, 81]. During the school and college stages, the students cope with sleep problems, such as sleep deprivation or sleep restriction, which produces a poor sleep quality and then academic problems [73]. The specific literature refers to a mean of 9 hours per night of sleep as a good requirement; however, 45% of the students sleep less than 8 hours per night and around 20–50% considers that they have sleepiness during the day [77, 78, 80]. The reasons of this time sleep reduction can be an interaction of intrinsic (e.g., physical and physiological changes induced by the puberty) or extrinsic (e.g., school schedules, extracurricular activities, family lifestyle, etc.) factors that lead to go late to bed with the daily routines

The consequence of this sleep loss is the sleepiness that the children and adolescents reflect during the day [68]. This fact reduces some neurobehavioral aspects of learning and memory, such as reduced alertness and decision-making, or difficulties when performing complex tasks (i.e., that require the use of the prefrontal cortex) during planning, integrative processes or abstract thinking [80, 91]. According to this rationale, the sleep loss appears during the stages from school to the university affecting the learning process and the academic performance in the different lessons (i.e., practice, laboratories, theory or exams) received by the

The physiological explanations of sleep loss are the changes that occur during the sleep-wake cycle caused by going to bed late and waking up early in the morning. The sleep-wake cycle is composed of two opposing processes: the circadian rhythm (described as a natural clock synchronized with the external time and regulates the wakefulness) and the homeostatic drive for sleep (process that increases sleepiness with the accumulated time awake and decreases when sleeping) [79]. The changes of this cycle appear with greater intensity during the adolescence with changes in the students' chronotype due to the delayed phase preference [80]. This delay is not only social, cultural or psychological [79, 81] and is affected by biological

**3.2. Sleep habits and academic performance in children and adolescents**

*3.2.1. The importance of sleep-wake cycle and the delayed phase preference*

unchanged [90].

students [84, 85, 88].

#### *2.3.3. Body fat*

Obesity during school-age period has been proved to have detrimental effects on academic performance [5, 60]. However, high percentage of body fat does not hamper, by itself, the likelihood of academic success. In a recent study over 11,192 kindergartens, achievement scores were significantly lower in overweight children than in no overweight children in standardized tests of mathematics and reading. However, this association was no longer significant after the adjustment to race/ethnicity and socioeconomic status [9]. In another study upon 36,870 adolescents, it has been shown that physical activity can attenuate or even counteract negative association of fatness on academic achievement. In particular, fit adolescents (both high and low fatness) had higher odds for attaining high academic scores (in language and mathematics) than high fatness unfit counterparts. Also, low fatness adolescents were not more likely to reach higher scores in language than their high fatness fit peers [54]. Thus, physical activity seems to strongly mediate between obesity and academic achievement. Although the evidence of this mediator effect is limited, a promising research in this topic is envisaged [60]. Furthermore, as some researchers consider [67] that the association between elevated body mass index and decreased academic achievement was dependent on the extent to which obesity was stigmatized in the school.
