key: cord-0024624-nnt9hf9l
authors: Ammar, Achraf; Boukhris, Omar; Hsouna, Hsen; Dhia, Imen Ben; Trabelsi, Khaled; Gujar, Tariq Ali; Clark, Cain C.T; Chtourou, Hamdi; Driss, Tarak; Hoekelmann, Anita
title: The effect of a daytime 60-min nap opportunity on postural control in highly active individuals
date: 2021-02-28
journal: Biol Sport
DOI: 10.5114/biolsport.2021.104067
sha: e861d0d57ded2ded1ca9abc9f08582b46b2467df
doc_id: 24624
cord_uid: nnt9hf9l

Although napping is commonly used as a strategy to improve numerous physical and cognitive performances, the efficacy of this strategy for improving postural balance has not yet been elucidated. Thus, the aim of this study was to conduct a comprehensive examination of the effect of a 60 min nap opportunity (N60) on different components of postural control. Ten highly active individuals (age = 27 ± 3.5 y, height = 1.75 ± 0.52 m, weight = 66.02 ± 8.63 kg) performed, in a randomized order, two afternoon test sessions following no nap (NN) and N60. Postural balance was assessed using the sensory organisation test (SOT), the unilateral stance test (UST), and the limits of Stability Test performed on NeuroCom(®) Smart Balance Master. The subjective rating of sleepiness before and after the nap conditions was also assessed. Compared to NN, N60 improved the composite balance score (p < 0.05, ES = 0.75, Δ = 5.3%) and the average and maximum percentage balance in the most challenging postural conditions of the SOT (p < 0.05 for SOT-4 and 5 and p < 0.0005 for SOT-6; ES range between 0.58 and 1.1). This enhanced postural balance in N60 was accompanied with improved visual (p < 0.05; ES = 0.93; Δ = 8.9%) and vestibular (p < 0.05; ES = 0.81; Δ = 10.5%) ratios and a reduced level of sleepiness perception (p < 0.001, ES = 0.87). However, no significant differences were found in any of the UST and LOS components’ scores (p > 0.05). Overall, a 60 min post lunch nap opportunity may be viable for improving static balance, although further work, involving larger samples and more complex motor activities, is warranted.

Sleep is a vital process with beneficial impacts on physical development, emotional regulation, and cognitive functions [1] , and plays an important role in molecular mechanism regulation [2] and metabolic homeostasis [3] . Waterhouse et al. [4] asserted that a night of good sleep is recuperative, produces an improvement in cognitive ability and removes the feelings of fatigue. Concordantly, Leeder et al. [5] reported that an adequate amount of sleep is crucial to achieve optimal performance and recovery. In contrast, night time sleep disturbance has been suggested to negatively affect endurance [6, 7] and short-term maximal intensity [9, 10] performances as well as reaction time, alertness and mood [8] .

Widely considered to be essential for optimal physical performance [11] , human balance and postural control have been 

Participants performed, in randomized order, two afternoon test sessions following no nap (NN) and a 60-min nap (N60) prior the measurement of postural stability, respectively. A minimum period of 48 h was observed between sessions. For the N60 condition, participants came to the laboratory at 12:45 p.m. and they attempted to take a 60-min nap from 1 p.m. to 2 p.m. in a quiet and darkened room.

To dissipate sleep inertia after nap opportunity, postural stability test was performed 60 min following wake up [21, 22] . For NN, participants spent the time between 1 p.m. and 3 p.m. reading books, watching videos on television, or playing video games in a comfortable armchair. To minimize the effect of diurnal biological variations, tests of postural balance occurred at same time of day (3 p.m.) in both conditions [32, 33] . The night before each experimental test, participants were asked to adhere to their normal sleep duration (~7 h).

The subjective rating of sleepiness before and after the nap conditions were assessed using the Stanford Sleepiness Scale (SSS). The SSS is a 7-point scale ranging from "1" (high activeness) to "7" (high tiredness) [34] . After completing the nap, participants were also asked about their subjective sleep quality using a scale ranging from 0" (no sleep), "5" (some sleep with some interruptions) to "10" (uninterrupted, deep sleep throughout) [26] .

Postural balance was assessed using the Smart Balance Master (Neu-roCom® International, Inc., USA) instrumented platform system. The device consists of a dynamic force plate, visual surround, overhead attachment for a safety harness strap and computer with software.

Both the force plate and visual surround are moveable. During all three measurements (i.e., sensory organisation, unilateral stance) and limits of stability tests), participants were asked to stand unshod on the force plate with hands resting on the iliac crests.

The Sensory Organisation Test (SOT) was employed to identify the sensory input influence during postural balance. The abilities as well as strategies to balance the posture were measured during six different conditions, which cause a suppression of the inputs from the inaccurate sensory system, and the participants generate appropriate motor and postural response strategies. This test compromises six conditions (SOT1-SOT6) as shown in Table 1 . Sway reference involve an anteroposterior rotation of the visual surround or platform (or both). Three trials were applied for each condition with a duration of 20 seconds/trial. Scores obtained after each condition were used to generate 4 scores associated with postural control: composite balance, and somatosensory, visual and vestibular ratios. These scores were calculated as follows: regular sleep has previously been recommended for healthy adults [17] , with an additional ~2 h sleep per night may be needed for athletes and students [18, 19] . However, previous reports indicate that sleep disturbances are frequent amongst athletes [20] and students, with ~ 70% of university students reporting sleep deprivation (6 to 6.9 h/night) [19] . Thus, it is essential for these populations to find an effective strategy to counteract the negative effect of sleep deprivation. In this context, napping has been suggested as a safe and non-invasive countermeasure to alleviate the consequences of nocturnal sleep deprivation [9] and overcoming the cognitive and physical deteriorations caused by sleep loss [21] . Nevertheless, studies investigating the effect of nap opportunity after normal (i.e., prophylactic naps) or disturbed (i.e., replacement naps) sleep on physical and cognitive performance have yielded inconclusive results.

Several studies have reported that various physical and cognitive performances, such as repeated-sprint [9, 22] , jumping [21] , endurance [24] , sports specific skills [25] , reaction time [23] , attention [21, 26] , and short term memory [27] , were improved by daytime napping. However, other reports failed to show similar efficacy [28, 29] .

Surprisingly, despite that postural control plays a key role in the performance of daily tasks, and contributes to achieve peak performance in many athletics activities (e.g., shooting accuracy, skating speed, precision, efficiency of martial art-specific techniques) [30] , the effects of napping on postural control has not yet been elucidated. Therefore, ascertaining the impact of diurnal napping on the ability to maintain body balance represents an important consideration [31] . Accordingly, the present study sought to investigate the effect of 60 min nap opportunity on different components of postural control (e.g., double leg and unilateral standing balance, sensory organization and limit of stability) during stable and challenging postural conditions.

Ten highly active sports science students (age = 27 ± 3.5 years, height = 1.75 ± 0.52 m, weight = 66.02 ± 8.63 kg, training experience ≥ 3 years) volunteered to participate in this study. All participants had normal or corrected-to-normal vision, and they reported no history of musculoskeletal or neurological disease, falls, dizziness, or complaints of vertigo. After receiving a detailed explanation about the study design, aims, and benefits, participants gave their written informed consent to participate. The protocol was approved by the local review board and the study was conducted according to the declaration of Helsinki.

The required sample size was calculated a priori using the G* power software (version 3.1.9.2; Kiel University, Kiel, Germany). Values for α were set at 0.05 and power at 0.80. Based on the study of Boukhris et al. [26] and discussions between the authors, effect size was estimated to be 0.86. The required sample size was therefore 

The Limits of Stability Test (LOS) assesses the participant's ability to intentionally displace the COG in eight directions (four cardinal and four diagonal directions, Figure 2 ) and to concurrently maintain stability at those positions without losing balance [36] . Participants performed the test while watching a real time display of their COG.

They were instructed as follows:" When you hear the tone and see In each condition, three trials of 10-second/trial were performed.

The right (condition 1 and 2) or the left (condition 3 and 4) foots was lifted to a standard height of 10 cm. During each trial, the center of gravity (COG) sway velocity (described as swapped degrees (θ) per second, figure 1) 

The balance percentages during the 6 conditions are presented in Table 2 . There was no significant difference between NN and N60

for both average and maximal postural balance from condition 1 to condition 3 (p > 0.05). However, the average and maximal postural balance increased after N60 compared to NN during the condition 4, 5 and 6 (p < 0.05).

Concerning the SOT's composite balance score (Figure 3) 

Although napping is commonly used as a strategy to supplement nighttime sleep in order to improve human physical and cognitive performances [21, 22, 26] , to the authors' knowledge, the present study is the first to evaluate the effectiveness of napping (i.e., N60) on different components of postural balance. Accordingly, the main findings of the current study were that N60 improved the composite balance score, and the average and the maximum percentage balance in the most challenging postural conditions (SOT4-6), as well as the visual and the vestibular sensory systems during the SOT. However, this napping opportunity only generated minor, non-significant, improvements in the postural sway velocity during the UST and in the RT, and MVL components of the LOS test.

The complex integration of the central nervous system with the visual, vestibular, proprioceptive, and musculoskeletal systems is widely considered as the basis for maintaining postural control and achieving or restoring a state of balance in everyday functional tasks [13] . Under stable conditions, balance ability mainly rely on the somatosensory system, with a high relative weight of 70% [37] .

However, on unstable surfaces, individual sensory dependence is (p < 0.05; Cohen's d = 0.93; Δ = 8.9% for the visual ratio; p < 0.05; Cohen's d = 0.81; Δ = 10.5% for vestibular ratio).

As shown in Figure 5 , there was no significant difference between NN and N60 in the postural sway velocity at any of the UST's conditions (p > 0.05).

Limits of stability scores are presented in Sleepiness perception was lower after N60 (p = 0.005, −55.2%) compared to NN ( Figure 6 ). In addition, sleepiness perception recorded after N60 was lower than after NN (p = 0.005; Δ = -48.3%). A daytime 60-min nap opportunity & postural control altered, with increases in the dependency on vestibular (60%) and visual (30%) information, concomitant to decreases in surface somatosensory inputs for postural orientation [37] . Importantly, under more challenging conditions that result in inter-sensory conflict (e.g., both the surface and visual environments are unstable), the dominance of the vestibular system is more pronounced [37] . Taken together, the ability to maintain postural control in challenging sensory contexts (i.e., changeable conditions with redundant inputs) seems to be dependent on the ability of the central nervous system (CNS) to quickly reweight sensory dependence and select the appropriate inputs to rely on to generate an appropriate motor response [38] . Therefore, it has been posited that interventions or strategies which yielded a beneficial effect on brain functions, such as daytime napping [39] , would also likely improve postural control.

Indeed, using different balance conditions, under stable and unstable support surfaces, with stable and/or sway visual surround, the present findings confirm this hypothesis, and indicate that N60 had a significant beneficial effect on postural balance during SOT's condition 4, 5 and 6. Moreover, these findings revealed that the visual and vestibular systems are positively affected by N60 and suggest that these systems have higher relative sensory dependence weights compared to the somatosensory systems during the more challenging conditions (SOT4-6). The results of the sensory systems ratios confirm these suggestions and showed significantly higher visual and vestibular ratios during N60 compared to NN. Regarding the overall SOT score, the present findings also showed an improved SOT's composite balance score during N60. This enhanced balance performance, compared to NN, may be attributed to a better integration of the central nervous system with the visual, vestibular and musculoskeletal systems [13] during the 3-last conditions (SOT4-6) of the SOT performed following N60.

Indeed, it has been previously demonstrated that there is an interaction between the sleeping or waking state of the brain and all sensory systems (i.e., visual, auditory, vestibular, somesthetic and olfactory) [40] . Particularly, the sensory information entered through the receptor may change the sleep-wake physiology, and conversely, the sleeping brain imposes rules on the incoming information [40] .

In this context, it has been reported that the reduction in vestibular system efficiency and/or the integration of the various sensory inputs could explain the perturbation of postural sway after sleep restriction [41] , which confirmed the reciprocal interactions between sleep and balance. Additionally, worse postural control performance has been previously reported as a consequence of bad sleep quality [42] .

In the present study, the level of sleepiness perception was lower after napping (i.e., N60) compared to NN. Indeed, these findings support previous studies which reported that napping was efficacious in reducing daytime sleepiness [26, 27] , and thereby improving postural balance. Regarding the underlying mechanism, the effectiveness of N60 on postural balance is most likely due to the occurrence of the slow wave sleep (SWS) -also known as deep non-rapid eye movement (NREM) sleep -during the nap. Indeed, NREM sleep has a vital role with restorative benefits for cognition [43] and is also associated with memory consolidation, learning of motor skills [25, 44] and improved physical [9, 21, 22, 24] and cognitive performance [21, 26, 27] performances. Additionally, SWS is thought to play an important role in cerebral restoration and recovery [45, 46] through a notable release of growth hormone [47] , restoration of physical damage (i.e., stress to bones, muscles, tissues, and organs) and reduction of stress and anxiety [26] . Therefore, it is not surprising to find an enhanced balance performance following N60, and plausible that the SWS contained in N60 would facilitate the better integration of the central nervous system with the visual, vestibular, and musculoskeletal systems [13] during SOT.

Concerning UST and LST, the current study did not observe a sig- Additionally, the effect of napping duration (i.e., longer vs. shorter durations) on postural control should be tested after restricted night sleep which was a common situation during COVID-19 home confinement [51] .

To the authors' knowledge, the present study is the first to investigate the effect the effectiveness of 60 min napping opportunity (i.e., N60) on different components of postural balance in healthy trained student.

The present study showed that, following a normal night sleep, a post lunch nap opportunity (i.e., N60) improved static postural control during challenging SOT's conditions under stable/unstable support surface with stable and/or sway visual surround. This enhanced postural balance was accompanied with improvements in the visual and vestibular sensory systems. However, N60 showed less efficacy on vigorous balance during UST and LST, which require more muscular efficiency and highly flexible postural control. Consequently, N60, or potentially longer napping durations (i.e., N90 allowing a complete sleep cycle), may be introduced before afternoon training sessions or late afternoon competition, in order to improve postural control which is a fundamental skill required to perform most of daily and sports activities. However, the veracity of these findings must be avowed in further studies.

However, the absence of an objective measurement of the sleep quality and the exact nap quantity (e.g., polysomnography) could limit the final conclusion of the present study. Moreover, although novel, the present study consisted of a small sample size, which precluded out ability to accurately detect small differences, despite the presence of moderate to large effect sizes in some cases. Indeed,

given the COVID-19 related restrictions and burden [52] , the data of the present study has been collected on only 10 participants.

Therefore, it could not be systematically generalized to athletics or students' populations and larger studies still needed to confirm the present findings. However, we present encouraging results that can be used a as platform to further understand the efficacy of napping on postural control.

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