Considerable attention has been paid to napping as an effective countermeasure for arousal decline and for improving cognitive performances. The previous literature has demonstrated that naps less than 30 min undermine the deleterious impacts of sleep deprivation that affects arousal and performances (1–3). It has also been reported that naps enhance arousal levels even when the quality and the quantity of the previous nocturnal sleep are adequate (4,5). It would be of interest to mention that the differences in results found in previous researches depend on the diversity of factors that determine the benefits acquired when taking short naps. Indeed, factors, like the quality of prior nocturnal sleep, sleep architecture, and temporal placements of the nap during the day, may determine the extent of benefits gained through short naps (6). Likewise, a number of interindividual differences embodied in the experiences with napping (7,8), more specifically, individuals’ abilities to produce slow waves (9), the occurrence of sleep inertia (10), and other factors like tests’ characteristics may affect the benefits of naps (7). Compared with long naps (more than 30 min), short naps have negligible effects of sleep inertia upon awaking (10). Although a substantial number of studies on the benefits of napping were conducted outside of the sports sphere (11), a short nap has commonly been favored to be incorporated in an athlete’s daily life, as it presents a practical strategy in enhancing recovery when regarding the multiple training sessions performed by sportspeople (12).
Previous articles have focused on how circadian rhythms and prior sleep affect athletic performances; circadian rhythms affect physical abilities and aspects of cognitive functions according to the time of day, with optimal performances at the end of the afternoon (13). Prior sleep influences optimal outcomes since it has been reported that sleep extension improves performances (14), contrary to sleep deprivation which negatively affects the evening performances by decreasing the normal rise of athletic outcomes at that time (15). Noting that before an athletic event, nocturnal sleep has frequently been proved to be disturbed (16). Besides, athletes in individual sports are more affected than athletes in team sports (17).
Given the overall detriments caused by sleep loss, napping is used as a behavioral countermeasure that limits the decrement in performances (11,18). Unfortunately, few studies have dealt with the effects of napping on athletes’ cognitive and physical performances during daily trainings or before athletic events (11). To the best of our knowledge, only two studies have examined the effects of naps on athletic performances. The first study, Waterhouse et al. (11), has indicated that a postlunch nap improves alertness and aspects of mental and physical performances after a partial sleep loss. The second study, Petit et al. (19), has shown that napping has no effects on performances, in particular when assessing the effects of a 20-min postprandial nap on a short-term physical exercise in normal conditions.
It is relevant to better emphasize that athletic performances depend on the complexity and efficiency of the physical, psychomotor, and cognitive components required to perform a specific sporting task (20). Karate is a competitive sport with a high level of environmental uncertainty (21). Furthermore, karate athletes have to simultaneously perform a mechanical work task with a great physical exertion, and a high level of perceptual abilities (20). The need to offend and defend against the opponents’ actions, requires (i) a great level of physical fitness, especially strength, aerobic fitness, muscle power, and speed (22) and (ii) developed cognitive skills, especially rapid anticipation, decision making, and fast reactions (20,23). Additionally, the decrement in muscle performances and mental fatigue emerge as a major problem when bouts of intensive efforts are imposed on competitors especially that, during an official competition, competitors have to perform several matches on the same day (24). Noting that, several studies have provided insight into the negative effects of a fatiguing bout of exercise simulating the costs of athletic efforts on physical outcomes (25,26), and on cognitive performances as well (27). These effects are more noticeable under a sleep deprivation condition (28). In this regard, it is worth pointing that the Karate-Specific Test (KST) is a recent and valid field test for assessing karate-specific endurance-fitness level (29). Karate-Specific Test was relevant in this study because it (i) replicated the real karate combat pattern, and (ii) granted the opportunity to investigate the effects of sleep deprivation and/or nap that have never been assessed with this kind of test.
Therefore, the purpose of this study was to examine the effects of a postlunch short nap, after a full night sleep, on subjective alertness and fatigue, and aspects of mental and physical components. In a perspective of performances’ optimization, we also aimed to identify which aspect of the cognitive and/or physical performances is/are affected by sleep loss, and could be recovered after a 30-min nap. Likewise, we aimed to investigate the effects of a nap on fatigue induced by a high-intensity intermittent specific karate test.
Thirteen international-level male karate athletes (age, 23 ± 2 yr; height, 175 ± 2.45 cm; mass, 68 ± 7.5; expertise, 15 ± 3) were volunteered to participate in the study. They were fully informed about the study information. Before participating, a written informed consent was obtained from every participant. The selective criteria for the participants were the following: subjects kept standard times for sleeping habits (bed time, from 10:30 pm to 7:00 am ± 1 h). All of them were habitual nappers. They were nonsmokers and nonalcoholic or caffeine consumers. They have been trained nine sessions of about 2 h. Since circadian typology might affect the study outcomes, the selection of the players’ chronotypes was based on the Horne and Ösberg self-assessment questionnaires (30). According to their answers to the questions related to their timing of sleep and their daily activities, subjects who presented an extreme morning or extreme evening type were eliminated. Instead, players who were “neither type” were involved in this study. The protocol of this study complied with Helsinki’s declaration for human experimentation, and was approved by the University Ethics Committee.
The experimental design is described in Figure 1. Two weeks before the beginning of the experimental period, the participants were familiarized with the equipment and the experimental procedures to minimize the learning effects during the course of the study. During habituation sessions, subjects took a short bout of nap in the laboratory. Thereafter, in experimental sessions, athletes were randomly assigned to experience both nap and no-nap conditions, after either a full night sleep (RN) (>7 h), or a partial sleep deprivation (PSD) (sleep 11:00 pm to 3:00 am). On the day before the experimental session, subjects came to the laboratory in the evening and had the same standardized dinner at 8:00 pm, and later they went to bed at 10:30 pm. The nocturnal sleep and naps were monitored by actigraphic recording and sleep diaries. In addition, they were strictly supervised by two experimenters during experimental nights.
To keep the subjects awake during the partial-sleep deprivation condition, they were allowed to watch television, read books, listen to the music and use computers. The four protocol conditions were carried out with at least 1 wk apart. This period allowed them to have a sufficient recovery from one night of sleep loss. The nap condition included a 30-min nap period from 1:00 pm to 1:30 pm in bed after lunch. In the no-nap condition, participants were free to choose their activities. A 30-min period separated the nap or the rest condition from the assessment sessions. Postnap testing took place in the National Center of the karate team of Tunisia. The postnap testing session started at 2:00 pm, first by recording the visual analogue scales to quantify subjective alertness and fatigue. Then, it consisted of the following tests, respectively, before and after the KST:
- Cognitive performance tasks: simple reaction time (SRT), mental rotation test (MRT), lower reaction test (LRT).
- Physical assessment session: squat jump, (SJ), counter movement jump (CMJ), KST.
Before the beginning of the physical testing session, subjects completed a standardized warm-up of 10 min, as recommended by Tabben et al. (29), and it consisted of self-selected-intensity jogging, vertical jumping, and dynamic stretching (hip extensors, hamstrings, hip flexors, and quadriceps femoris), and later followed by a 5-min passive rest. The recovery time between every physical test was about 5 min, and 2 min between every trial in the same test.
Different precautions were taken before the testing session. Instructions about sleep and diet were given to the subjects. During the period of investigation, they were prohibited from consuming food, beverages, or any known stimuli (e.g., caffeine) or depressants (e.g., alcohol) that would possibly enhance or compromise alertness. Throughout the experimental period, participants were requested to maintain their habitual physical activities, and to avoid strenuous activities during the 24-h before the testing sessions. It was easy to control compliance with these directions because the participants were students in the same institution with the same daily schedules. The protocol of this study was made to be close enough to athletes’ typical real life routines.
Actigraphy and sleep diaries
For objective sleep measurements, we used the MotionWatch 8© actigraphy system (MW8; camntech). To ensure that subject’s sleep patterns throughout the course of the study was typical (i.e., not unusually restricted or extended), actigraphic recording was edited with information listed in the subjective sleep diaries. The MW8 data was downloaded and analyzed using MotionWare version 1.0.25 (camntech). A high-level sensitivity was used for the estimation of sleep-wake patterns in 60-s epochs.
To make sure that the protocol of the study was fully respected, the following sleep parameters of the night before the experimental session were analyzed. The parameters were: Bed time, Fell asleep time, Woke up time, Total time in bed, and Assumed sleep (the total elapsed time is between the “Fell Asleep” and “Woke Up” times).
Subjective alertness and fatigue
Subjective sleepiness and fatigue were evaluated using the 100-mm-long visual analog scale (31). The values ranged from 0 (very alert or in peak physical condition) to 100 (very sleepy or very tired).
Simple reaction test
When performing the reaction test, subjects were asked to response as quickly as possible to a visual stimulus presented systematically at the center of the computer screen. Participants were asked to press a button with the index finger when a visual stimulus (Blue Square) appeared. Simple reaction test was conducted using software Reaction, INRP free software (version 4.05) created by Tilquin (32).
Mental rotation test
Participants were asked to compare two of the four stimuli samples presented on the screen and to state if they are identical, except the rotation, or are different from the reference figure presented on the left of the screen. The stimuli used in this test are based on Shepard and Metzler stimuli (33). The test required mental maintaining and manipulation of the presented figures with accuracy and speed. There were 20 items in the set, and 3 min were given for each subset of 10 items separated by a 2-min break. We used the same procedure as Peters et al. (34) for scoring, giving one point for each item only if both answers were correct. This test was also conducted using software Reaction, INRP (version 4.05) created by Tilquin (32).
Lower body reaction
The simple acoustic-reaction time of the lower limb was measured using an optical jump system (Optojump Next, Microgate, Italy). The trial started inside the testing area. When receiving the acoustic sound, the subject must jump as quickly as possible. The sound was randomly generated by the measuring system and every stimulus lasted 1 s. Then, the Optojump system recorded the delay in time between the beginning of stimulus and the flight time (Time when the athletes left the testing area by jumping).
SJ and CMJ
The CMJ included a leg flexion from the upright standing position with the hands on the hips and immediately followed by a quick descended position to 90° knee flexion, and instantly performed an explosive concentric action for maximal height with the hands on the hips. The SJ consisted of a maximal jump held from a flexed knee position (approximately 90°) with the hands on the hips and without performing any countermovement before the beginning of the jump. These jump tests were monitored with an optical jump system (Optojump Next, Microgate, Italy). The apparatus comprises two bars placed ~1 m apart, parallel to each other, and was interfaced with a microcomputer via a USB port. The optical system transmits an infrared light 1–2 mm above the floor. When the light is interrupted by the feet, the units trigger a timer with a precision of 1 ms which allows the measurement of flight time and contact time. For both tests after the signal, the subjects started whenever they felt ready to jump but not more than 4 s for SJ test. The participants performed three trials in every test with recovery time of 2 min between trials, and finally, the best one was recorded.
The test protocol consisted of sequential sets composed of two attacks toward a body opponent bag. The first attack included a two-punch combination, lead straight punch followed by a rear straight punch (i.e., kisamigyaku-zuki), and the second attack used a rear roundhouse kick (i.e., mawashigeri-chudan). The test was packed with two auditory signals; the first was in the beginning of the bout of the exercise, and the second sound indicated the rest time. The time to complete the exercise bout remained the same for 3 s; however, the recovery time between bouts progressively decreased every 3 min. Every punch and kick had to be executed with the maximum power possible. The test stopped and the time to exhaustion (TE) was determined when the participants failed to complete the set of movements in the 3-s interval twice, or when there was a clear decrease in force or in performed techniques. They performed the KST wearing protections that are specifically designed for karate. The test was carried out on a tatami (i.e., competition karate floor). Karate athletes must use their favorite guards and were not allowed to change their guard during the test. Distance between the body opponent bag and the front foot was established and fixed before the beginning as 1.5 m.
Statistical tests were processed using Statistica 7.1 (Statsoft, France). Data were reported as mean ± SD. Paired t-tests were used for sleep variables between the control and the nap conditions after RN, and the control and the nap conditions after PSD. The data concerning the effects of napping on the performance was conducted using a two-way ANOVA analysis [2 (nap conditions) × 2 (sleep conditions)] with repeated measures. Data concerning the effect of napping on fatigue induced by KST was conducted using a two-way ANOVA analysis [2 (nap conditions) × 2 (sleep conditions)] expressed by calculating the difference between the performances after and before KST. When the ANOVA indicated significant main effects, differences between the means were tested by Tukey’s post hoc tests. The level of statistical significance was set at (P < 0.05).
Prior Nocturnal Sleep
The actigraphic data indicated that equivalent amounts of five sleep parameters (e.g., bed time, fall sleep time, wake up time, time in bed, assumed sleep) obtained on the night preceding testing under both the nap and no-nap conditions following a reference night and partial sleep deprivation night (see Table 1).
Subjective Alertness and Fatigue
There was a significant main effect nap (F(1–12) = 5.14, P < 0.05; ηp2 = 0.30), indicating that subjective alertness increased after the nap comparing to the no-nap condition. Moreover, there was a significant main effect for sleep (F(1–12) = 66.50, P < 0.001; ηp2 = 0.84), indicating that sleep loss decreased the alertness level comparing to the reference night. However, there was no significant main effect for nap–sleep interaction (P > 0.05) (see Table 2).
The results showed that there was a significant main effect sleep (F(1–12) = 85.69, P < 0.001; ηp2 = 0.87), indicating that subjective fatigue increased after the partial-sleep deprivation condition comparing to the reference night. However, there was no significant main effect for nap (P > 0.05), and for nap–sleep interaction (P > 0.05) (see Table 2).
The effects of napping on performances
Simple reaction time
There was a significant main effect of nap (F(1–12) = 5.34, P < 0.05; ηp2 = 0.30), indicating the improvement in SRT performance after the nap compared with the no-nap condition. However, there was no significant main effect for sleep (P > 0.05), and for nap–sleep interaction (P > 0.05) (see Table 3).
Mental rotation test
There was a significant main effect of nap (F(1–12) = 5.03, P < 0.05; ηp2 = 0.29), indicating that the score in this test was better after the nap compared with the no-nap condition. Moreover, the results showed a significant main effect for sleep (F(1–12) = 5.28, P < 0.05; ηp2 = 0.30), stating that the performance decreased after the partial sleep deprivation compared with the reference night. However, there was no significant main effect for nap–sleep interaction (P > 0.05) (see Table 3).
Lower reaction test
There was a significant main effect of nap (F(1–12) = 6.86, P < 0.05; ηp2= 0.36), with an increase being observed after the nap compared with the no-nap condition. Moreover, the results revealed a significant main effect for sleep (F(1–12) = 4.69, P < 0.05; ηp2 = 0.28), showing that the performance decreased after the partial sleep deprivation compared with the reference night. However, there was no significant main effect for nap–sleep interaction (P > 0.05) (see Table 3).
The effects of napping on fatigue induced by the KST
Simple reaction time
The results showed that there was no significant main effect for nap (P > 0.05), sleep (P > 0.05), and for nap–sleep interaction (P > 0.05) (see Table 4).
Mental rotation test
The results showed that there was no significant main effect for nap (P > 0.05), sleep (P > 0.05), and for nap–sleep interaction (P > 0.05) (see Table 4).
Lower reaction test
There was a significant main effect for nap–sleep interaction (F(1–12) = 5.58, P < 0.05; ηp2 = 0.31). However, there was no significant main effect for nap (P > 0.05), and for sleep (P > 0.05). The post hoc test revealed that the effect of nap on fatigue after the KST was depended on the sleep condition. In fact, in the partial sleep deprivation condition, performance was the same with and without napping (P > 0.05). However, following the reference night performance was better with napping than without napping (P < 0.05) (see Table 4).
Time to exhaustion during the KST
There was a significant main effect for nap (F(1–12) = 10.88, P < 0.01; ηp2 = 0.47), sleep (F(1–12) = 10.37, P < 0.01; ηp2 = 0.46), and for nap–sleep interaction (F(1–12) = 8.98, P < 0.01; ηp2 = 0.42). The post hoc test demonstrated that the nap improved the time to exhaustion after the partial sleep deprivation night (P < 0.001). However, napping did not improve the performance after the reference night (P > 0.05). Regarding the effect of sleep deprivation, during the no-nap condition, time to exhaustion was affected by sleep loss compared with the control condition (P < 0.01). After napping, the effect of sleep deprivation disappeared compared with the reference night (P > 0.05) (see, Fig. 2).
The effects of napping on performances
The results showed that there was no significant main effect for nap (P > 0.05), sleep (P > 0.05), and for nap–sleep interaction (P > 0.05) (see Table 3).
Counter movement jump
There was a significant main effect of sleep (F(1–12) = 7.04, P < 0.05; ηp2 = 0.37), indicating that the partial sleep deprivation significantly decreased the performance in CMJ compared with the reference night. However, there was no significant main effect for nap (P > 0.05), and for nap–sleep interaction (P > 0.05) (see Table 3).
The effects of napping on fatigue induced by the KST
There was a significant main effect nap (F(1–12) = 8.43, P < 0.01; ηp2 = 0.41), indicating an improvement in SJ performance flowing the nap comparing with the no-nap condition. However, there was no significant main effect for sleep (P > 0.05), and for nap–sleep interaction (P > 0.05) (see Table 4).
Counter movement jump
There was a significant main effect nap (F(1–12) = 6.15, P < 0.05; ηp2 = 0.33), indicating the improvement in CMJ performance flowing the nap comparing with the no-nap condition. However, there was no significant main effect for sleep (P > 0.05), and for nap–sleep interaction (P > 0.05) (see Table 4).
This study is the first in our knowledge that aims to evaluate the effects of a 30-min nap after lunch on subsequent alertness, fatigue, cognitive and physical performances in karate athletes after two nocturnal regimens (i.e., RN and PSD). Our results have revealed that a 30-min nap after RN has led to significant improvements in alertness and in cognitive performances. However, there has been no significant change in physical performances. Indeed, our results have demonstrated that a PSD has led to a significant alteration in alertness, fatigue, as well as in cognitive (MRT, LRT) and physical (KST, CMJ) evening performances. Nevertheless, the 30-min nap has restored the decrement in performances caused by sleep loss in most of the performed tests. Regarding the effects of napping on fatigue induced by the KST, the results have revealed that short naps have had ergogenic effects on physical performances (CMJ, and SJ) and a partial psychogenic effect on cognitive performances (LRT).
Compared with no-nap condition, the results of the present study have revealed that after RN, the 30-min nap has significantly improved alertness and cognitive performances (i.e., SRT, LRT, and MRT). These results are consistent with the previous reports indicating that short naps produce a wide range of benefits from increasing alertness to improving cognitive outcomes (4–6). What is noteworthy in this study is the noticeable improvement in response times as assessed by the SRT and LRT. The gain acquired in the information processing speed is relevantly meaningful to an elite athlete in the context of high-level sports. Therefore, these findings seem to be of great usefulness when considering that a small change in the context of highly professional levels can make all the difference. However, no significant change has been observed after the nap on fatigue and on physical performances (i.e., SJ, CMJ, and KST). These results are in line with Petit et al. (19) study that has demonstrated that napping has no beneficial effects on physical performances. In this context, it is important to mention that the absence of effects on physical performances may also indicate that the period of 30 min that separates the nap from the assessment session is sufficient to avoid any negative effect of sleep inertia (11). Evidently, optimal alertness levels and cognitive skills are integral parts of the abilities required to reach the highest elite-sport performances. Indeed, the present findings indicate that napping is an efficient strategy that needs to be taken into consideration among elite sportspeople in their pursuit of excellence, especially when napping does not violate doping regulations.
Along with the present study, the results have also indicated that PSD leads to a significant decrease in subjective levels of alertness and fatigue. In the same way, the scores in MRT and LRT are negatively affected by sleep loss. These findings are consistent with previous studies attesting that sleep deprivation has negative impacts on subjective alertness, fatigue, and cognitive performances (28,35). However, no significant change is observed in SRT and this might be due to the limited duration of this test (35).
Regarding the effects of partial sleep deprivation on the physical performances, the results have showed a decrease in CMJ, while no change in performance is observed in SJ. These results can be explained by the fact that the CMJ has a higher sensitivity to detect neuromuscular fatigue compared with the SJ (36). Added to that, the results have revealed that time to exhaustion during KST is negatively affected by sleep deprivation. Noting that, the KST measures the maximal aerobic capacity of karate athletes (29). Therefore, the decrement in performance observed in this test may reflect the limited physiological capacity of sleep-deprived athlete to perform a physical work-load level. These results are in line with of Souissi et al. (15), and Bougard and Davenne (37) studies, that have revealed a decrease in the evening performances as a consequence of lower circadian rhythm amplitude caused by sustained wakefulness. Accordingly, the present data suggest that the impairments in Karatekas’ outcomes during competitions can be explained by the decrease in cognitive and physical evening performances caused by sleep loss. However, the decrease in performances is restored after the nap in most of the tests.
In other respects, the results of the present study have indicated that short daytime naps (i.e., 30 min) are effective to counteract the deleterious effects of partial-sleep deprivation on alertness and performances for high-level athletes. In fact, the impairment of time to exhaustion during KST is suppressed after the nap. This finding can be regarded as one of the main results of the present study. Likewise, the decrement in the performances observed in LRT and MRT is restored by the nap. These findings are congruent with previous researches showing that short naps have the possibility to restore the impairment in alertness and performance caused by sleep loss (1–3), and more specifically, the present data support Waterhouse et al. (11) findings attesting that a postlunch nap improves alertness and aspects of mental and physical performances after a partial-sleep deprivation. According to our hypothesis, these findings can be placed in a professional context by implementing napping as a strategy to overcome the drawbacks of performances’ deterioration caused by sleep loss especially for high-ranking athletes. However, the nap does not restore the decrease in subjective fatigue and the performance in CMJ test. A possible explanation is that a short nap opportunity for highly active athletes is not a sufficient duration to restore subjective fatigue and the performance in CMJ test.
In the current study, the results have revealed that the nap reduces performances deficits caused by KST in SJ and CMJ tests. Indeed, the decrease in LRT performance depends on sleep conditions. In fact, the nap restores the performance in this test only after RN. These results can partially confirm to Pelka et al. (38) study, which examines the effects of different relaxation strategies on physical performances in terms of recovery between two equal sprint protocols. Their results have demonstrated that the use of breathing regulation and a power nap leads to improved running speed comparing to the control condition, yoga and progressive muscle relaxation. A variety of strategies have been proposed to have an ergogenic potential for athletes such as caffeine consumption (39), light exposure modalities (40), and more recently, the cupping therapy (41). However, there is no research available on napping as a strategy to counteract performances’ deterioration caused by fatigue in a context of high level sport. Let us recall that fatigue in this study was induced by a specific intermittent field test simulating the karateka’s maximal physiological exertion (29) and, the present results prove that short naps have ergogenic and partial psychogenic potentials for athletes, as it can be considered as an effective strategy for the management of fatigue.
In summary, the present results have demonstrated, first, that napping is a potential strategy that ensures better performances levels among elite sportspeople in their pursuit of excellence. Second, short daytime naps are effective to counteract the deleterious effects of partial sleep deprivation. Again, these findings can be placed in a professional context by implementing napping as a strategy that aims at managing the fatigue induced by the exercise. Noting that, these results may suit the professionals’ goals, and without violating doping regulations. Nevertheless, future researches are required to give insight into the neurophysiological mechanism responsible for the improvement of the athletic performance gained from such naps.
This research was financially supported by the laboratory UMR 1075 INSERM/Unicaen, COMETE. Houda Daaloul was involved in drafting and revising the manuscript. Damien Davenne and Nizar Souissi were the investigators on the project and were involved in the design of the study, the statistical analysis and interpretation of the data, and manuscript preparation. The experiment elaborated in this study was held by Houda Daaloul and Hamdi Belhassen. The authors are thankful for the principal coach and the physical trainer of the national team of karate in Tunisia. The authors are also thankful to all the participants for their maximal effort and their contribution. No conflicting financial, consultant, institutional, or other interests exist. Results of the present study do not constitute endorsement by the American College of Sports Medicine. The results of this study are presented clearly, honestly, and without fabrication, falsification, or inappropriate data manipulation.
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