The Effects of Physical Activity on Sleep among Adolescents and Adults: A Narrative Review

Objective: Examining the correlation between physical activity measures and sleep in normal adolescents and adult population. Methods: A systematic review was conducted on the effects of exercise, its intensity, its frequency and its timing and sleep outcome. Using the databases including MEDLINE, PsycINFO and SPORTDiscus, keywords used were “sleep”, “circadian rhythm”, “exercise”, and “physical activity”. Results: To improve the general quality of sleep amongst adolescents and adults, individuals should engage in physical activity at any time of day. It also appears very advantageous to engage in long-term physical activity to maintain the positive effects on sleep. Nonetheless, it seems that sedentary individuals can also benefit from occasional physical activity to improve the quality of their subsequent sleep period. Conclusion: A virtuous relationship between physical activity and sleep may exist in normal adolescents and adults. People should seek to benefit from this link to improve both the quality of their sleep and of their daily functioning. However, larger scale studies, controlling for variables might help to better delineated this relationship. DOI : 10.14302/issn.2574-4518.jsdr-18-2065 Corresponding author: Sophie Desjardins, Department of Psychology, Universite du Quebec a Trois-Rivieres, Canada. Email: sophie.desjardins@uqtr.ca


Introduction
The American Academy of Sleep Medicine [1] maintains that physical activity is a highly effective tool of good sleep hygiene. Many authors [2][3][4][5] share this view.
The World Health Organization [6] defines physical activity by type, frequency, intensity, duration and total quantity. These variables are often retained during studies of physical activity and sleep. However, most studies do not control for all of these variables simultaneously. The findings are consequently less conclusive because specific covariates need to be examined in addressing the relationship between physical activity and sleep.
Another variable of physical activity that may influence sleep, not used by the World Health Organization, is the time of day when it occurs. Some authors [7,8] argue that physical activity performed before the sleep period does not influence the quality of sleep, whereas other researchers report opposite results [9]. These results will be reviewed in this paper.

Links between physical activity and sleep
No meta-analysis exists about the links between physical activity and sleep in the adolescent population.
However, regarding the adult population, three meta-analyses [10][11][12] were performed before the 21st century. The meta-analysis of Kubitz et al. [11] examined 32 studies of acute exercise and 12 studies of regular exercise, while the meta-analysis of Youngstedt et al. [12] reviewed 38 studies of acute exercise.
The latest meta-analysis [10]  Results also suggested that regular exercise has effects on TST (d = 0.25, p = .005), SE (d = 0.30, p = .002) and SOL (d = -0.35, p < .05), indicating that individuals who participated in regular exercise training had significantly greater TST and SE and shorter SOL than individuals in control conditions. Effects were moderated by sex, age, baseline physical activity level of participants, as well as exercise type, time of day, duration, and adherence. Significant moderation was not found for exercise intensity, aerobic versus anaerobic classification, or publication date.
The generalization of these findings to adolescents is uncertain as adolescent sleep and circadian rhythms may differ from that of adults.
Adolescence is a period of maturation and development in which biological and environmental factors are likely to cause changes in sleep patterns [13]. For example, percentage of stage 2 sleep increases with age from childhood until old age [14]. It is also known that SWS decreases across adolescence by 40% from preteen years and continues a slower decline into old age, even when length of nocturnal sleep remains constant [15].
Moreover, adolescents are recognized to have a biological delay in the timing of sleep onset, which can result in them staying awake later [16].
Given the above, the purpose of this narrative review is to present the studies that link physical activity and sleep in adolescents and adults within the normal population. Unlike previous meta-analyses, the following review: (1) includes adolescents and adults, (2)  x minute -1 for males and females, respectively. "Very fit" subjects are competitive endurance athletes [21].

Results
The set of results obtained from the articles retained will be discussed in this section.   Overall, it seems that physical activity performed at a sustained intensity has enduring positive benefits regarding the sleep period. Moderate physical activity may also be beneficial, but the findings are very heterogeneous, even less supportive to low intensity exercise. It is also important to mention that the frequency of physical activity also seems to explain much of the results obtained.

Frequency of Physical Activity
Kalak et al. [42] found that regular physical activity is a factor that favours better subjective and

Intensity of Physical Activity
Some studies conclude that light, moderate or sustained physical activity has no negative or positive effect on the sleep period [9,39,61]. In contrast, Tatum [55] reports a positive association between SE and intensity of physical activity.
Several studies report benefits related to the intensity of physical activity. Moderate activity was associated with better SE [7], shorter SOL [52], less time spent in the first stage of sleep [7] and fewer NAASO [7]. In addition, an increase in the proportion of SWS in the first cycle of sleep and better subjective sleep quality were observed [60].  [55] observed a positive association between frequency of physical activity and SE, which highlights the importance of the frequency variable.
In short, it appears very advantageous to engage in long-term physical activity to maintain the positive effects on sleep. Nonetheless, it seems that sedentary adults can also benefit from occasional physical activity to improve the quality of their subsequent sleep period.

Physical Fitness of Sleepers
Several studies show that physical activity amongst normal people has a positive impact on sleep: better SE, less time spent in the first stage of sleep, fewer NAASO [7,54], more SWS [54], and more time spent in the first four stages of sleep [8].
Amongst individuals considered as sedentary, physical activity promotes SE and reduced daytime sleepiness [53,60]. The latter change is attributed to an increase in SWS [53,60]. Amongst these participants, physical activity has other benefits, including an increase in the subjective quality of the sleep period [60], and a decrease in SOL [53]. Further, the decrease in SOL is also observed in another study of the same type of participants [52]. However, these results are not corroborated by Wong et al. [58], who observe only a minor improvement in the sleep parameters when sedentary individuals engage in physical activity, namely more time spent in the first two stages of sleep. It is possible that these divergent results arise from methodological weaknesses such as the number of participants, which was low in some studies [58,60].
Regarding physically active individuals, several authors [9,39,61]  In summary, benefits of physical activity on sleep appear more important in sedentary individuals compared to physically active adults. In return, stopping physical activity in the latter group may interfere with sleep.

Discussion
In general, given the results compiled in this article, physical activity seems to have a positive impact on both objective and subjective sleep quality.
Therefore, individuals who want to improve their sleep should adopt a physical activity program that considers the variables reviewed in this article. Specifically, the results of this review generally demonstrate that the proportion of SWS increases during the sleep period following physical activity. This is very beneficial for individuals who are physically active because it is during SWS that the growth hormone is secreted, which is responsible for the body's recovery [65]. However, exercise decreases REM which is where consolidation of memory [66] and emotional memory processing occur [67]. The decreased REM propensity may be due to an increased need for non-REM or alternatively an indicator of sleep fragmentation [68]. Another plausible mechanism for the alteration of REM episodes is the elevation of catecholamine levels, rather than some direct brain function response [69].
Researchers have observed that the frequency of physical activity amongst adolescents has a significant impact on sleep duration [45,47,48] what Kredlow et al. [10] argued but as Tatum [55] contends, it seems difficult to determine the effect of the type of physical activity on sleep because it may be confounded with the effect of the intensity of the physical activity. Therefore, Tatum

Conclusion
To improve the general quality of sleep amongst adolescents and adults in the normal population, the results of this article suggest that individuals should engage in physical activity at any time of day. Further, it seems that moderate to sustained physical activity must be practised at least twice a week for significant changes in sleep to be observed. It also appears very advantageous to engage in long-term physical activity to maintain the positive effects on sleep. Nonetheless, sedentary and normal individuals can also benefit from occasional physical activity to improve the quality of their subsequent sleep period. However, the benefits will endure only if the physical activity is regular.
Physical activity offers several health-related advantages [6,[77][78][79][80]. This is why its promotion is particularly important. In a similar vein, it is now established that prolonged sedentary behaviour, characterized by a sitting or reclining posture and low-energy expenditure, tends to be associated with an  [81].
Poor sleep, in turn, has several effects on the health of individuals and on society as a whole. It is associated with lower performance and productivity, higher rates of physical and mental disorders, increased accidents, substance use and abuse, mortality and morbidity, and elevated costs of prescriptions, therapies, diagnostic testing, and doctor visits [82].
Sleep and exercise influence each other through complex, reciprocal interactions including multiple physiological and psychological pathways [75]. People