This research utilized a counterbalanced, randomized experimental design to examine how varying periods of napping (40, 60, and 90 min) impact physical and physiological performance after PSD. Participants completed four experiment trials, three different nap periods and one of no nap, with at least a 72 h recovery period between trials. All assessments were conducted during pre- and post-nap periods.

16 male soccer players from the MASUM team of Universiti Pendidikan Sultan Idris (UPSI) were recruited to participate in this study. Participants age ranged from 20 to 24 years. Table 1 shows the demographic data of the participants. Through a purposive sampling technique, soccer players who have represented UPSI in MASUM-level competitions were selected since it warrants that respondents have gained experience, a level of skill, and familiarity with competitive soccer necessary for yielding valid and reliable information concerning the effects of naps on soccer-specific performance metrics.

A priori estimation of the sample size was calculated in relation to past studies directly about the use of repeated-measures designs and a subset of 10 to 20 subjects using a nap intervention on an athletic population.6,8 The number of 16 individuals who completed all the experimental conditions aspects satisfied and surpassed the sample size which is normally considered in similar research studies and gave the study enough statistical power to identify differences within a subject in regard to the nap conditions.

The inclusion and exclusion criteria for participants include:

Sleep and nap duration were objectively monitored using wrist-worn actigraphy (MotionWatch8, CamNtech Inc., USA), which has demonstrated acceptable validity for sleep–wake estimation in field-based sport research. One day before the trials, the participants fitted with ActiGraph (MotionWatch8, CamNtech Inc. USA) to record their sleep pattern to ensure their wakeup’s time consistently throughout the study. Activity during sleep and naps were recorded until the end of the study trials. To ensure that participants were sleep-deprived in this study, they were monitored by the researcher and placed in one room. All the participants were instructed to sleep from 3.00 am until 7.30 am, to prevent sleep before the time is allowed, they provided and allowed to watch movies and play video games. They also requested to avoid any strenuous activity for 24 h prior to testing and refrain from heavy meals, caffeine or nicotine within 2 to 3 h of testing.

Nap sessions were conducted in a controlled sleep laboratory environment with standardized ambient temperature, reduced light exposure, minimal noise, and researcher supervision to ensure compliance with assigned nap durations. On arrival, height and body mass were measured. The participants filled in the Pittsburgh Sleep Quality Index (PSQI) and received baseline physiological measurements. The standard warm-ups were then carried out, preceded by a series of trials: vertical jump, and 6 × 20 m RSA test. During the RSA test, Heart Rate (HR), Rating of Perceived Exertion (RPE), and blood lactate (Lac) were recorded.

Participants then received one of the four nap conditions. Naps were monitored using ActiGraph (MotionWatch8, CamNtech Inc., USA) worn on the non-dominant wrist. Post-nap, a 30 min buffer was allowed to minimize sleep inertia. The same physical and physiological tests were repeated after the nap. All post-nap performance tests were administered following a standardized 30 min wake buffer to minimize sleep inertia effects, and testing was conducted at the same time of day across all experimental sessions to control for circadian variation. The study assessed the effects of different nap durations on physical and physiological performance using validated protocols and standardized instruments as shown in Fig. 1.

Fig. 1.

Assessment Tool (A) VJ test. (B) RSA test. (C) Lac measurement using fingertip blood sampling with lactate pro analyzer.

Two primary tests were used to evaluate explosive power and anaerobic capacity:

Three physiological parameters were monitored during and after performance testing:

ActiGraph devices (MotionWatch8, CamNtech Inc., USA) were used to measure objective sleep and nap, whereas subjective sleep quality was determined by the Pittsburgh Sleep Quality Index (PSQI, Malay version). Nap interventions were carried out in an environment controlled at the Sleep Laboratory, Faculty of Sports Science and Coaching, UPSI. Pre- and post-nap testing was carried out by following a standard order, and participants were observed during the process to ensure adherence to partial sleep deprivation and nap procedures.

IBM SPSS Statistics was used to analyze data. Prior to analysis, datasets were screened for completeness, outliers, and adherence to normality assumptions using the Shapiro-Wilk test. A two-way repeated measures ANOVA was used to examine the interaction effects between nap duration (no nap, 40, 60, and 90 min) and time (pre- and post-test) on each dependent variable. Where violations of sphericity were detected, Greenhouse-Geisser (ε < 0.75) or Huynh-Feldt (ε > 0.75) corrections were applied accordingly. Post hoc pairwise comparisons were conducted using Bonferroni-adjusted significance levels to control for familywise error across multiple comparisons using the Bonferroni adjustment method. Statistical significance was set at p < 0.05.

Partial eta squared (η²) was calculated as a measure of effect size for ANOVA outcomes. Where applicable, 95% confidence intervals were estimated for mean differences. Assumptions of normality and sphericity were verified prior to analysis.

The study examined the effects of napping at 40, 60, and 90 min on the performance of athletes in high-intensity, short-duration activities, such as vertical jumps and sprints. CMJ tests are widely used to evaluate these parameters because they are sensitive to fatigue and recovery states. Significant improvements in jump height post 40, 60, and 90 min naps were observed for the CMJ test at 38.65 ± 3.89 cm, higher than post-no-nap conditions of 33.42 ± 4.38 cm (p < 0.001). In contrast, the 40- and 60 min naps showed significant improvements compared to the no-nap condition. These findings underscore the crucial role of sleep in promoting synaptic plasticity and muscle coordination, which are important for high-intensity performance. Muscle coordination is a important energy form for high-intensity activities with short durations, such as vertical jumps.17 Sleep deprivation suppresses glycogen synthesis but napping in 90 min blocks, enabling its replenishment and promoting muscle power and explosiveness.18 Further, the facilitation of neuromuscular recovery, motor memory consolidation, and increased muscle contraction efficiency has been achieved with deeper stages of sleep accessed during the 90 min nap, namely SWS and REM.18

These findings align with the work of Ayhan, who observed that a brief, 30 min nap following partial sleep deprivation significantly improved cognitive performance, short-term high-intensity physical output, and mood.17 Sleep deprivation suppresses glycogen synthesis but napping in 90 min blocks, enabling its replenishment and promoting muscle power and explosiveness.1 Further, the facilitation of neuromuscular recovery, motor memory consolidation, and increased muscle contraction efficiency has been achieved with deeper stages of sleep accessed during the 90 min nap, namely SWS and REM.18

In addition to improving CMJ performance, nap manipulations significantly improved all RSA parameters (TST, MST, BST, and FI). The significant improvements in performance were observed to occur after the 90 min condition. Overall, TST was reduced following the 90 min nap condition compared with the no-nap condition, suggesting faster sprints. However, the 40- and 60 min naps further contributed toward improvements in TST, though less pronounced compared to improvements instituted after a 90 min nap condition, at p = 0.001. These findings are supported by Souabni et al.19, who affirm that longer naps positively impact anaerobic activities because of energy and neuromuscular coordination facilitation.

Suppiah et al.20 reported that sleeping did not significantly affect mean times to sprint the 10- and 20 m distances or the best times to sprint 2 m and 10 m, respectively, from a standing position in adolescent athletes. In this context, the effect of napping on shuttle run performance showed no consistency; therefore, sleep can affect these physical parameters different.

The significant enhancement of the TST during the experiment follows the length of rest in every nap period, the rest for 90 min resulted in the highest increase. Much longer napping than this study resulted in significant positivity influencing sprint performances through reduced time, time spent in tension, and recovery rate improvements, although there is complete contradiction to this inference found by Suppiah et al.20. These results provide invaluable insight into sleep as an optimizer of athletic performance.

The results substantiate the effectiveness of a 90 min nap, a significant recovery intervention in mitigating impairment induced by sleep deprivation, thereby establishing the importance of providing an opportunity for restorative sleep in any context of PSD. For instance, improvements in jump CMJ and RSA following 90 min-napping in TST MST, BST, and FI revealed that rest contributed to mitigating muscle and perceptible fatigue levels for subsequent optimal performance.

This study found that longer naps provide a greater degree of physiological recuperation after exertion, particularly after a 90 min nap intervention. This supports previous studies that have demonstrated that adequate sleep plays a fundamental role in physiological recovery, particularly in cardiovascular and metabolic domains. Deep and REM stages of sleep are critical for autonomic rebalancing and the removal of lactate through glial cells. The findings also support the cardiovascular and metabolic recovery theory by Mah et al., which suggests that longer nap advances faster and more effective recovery after exercise, favoring overall athletic performance.3 The study also found that napping significantly reduced HRmax post-sleep deprivation, particularly after a 90 min nap intervention, which indicates a positive impact on cardiovascular recovery. This lowered HRmax indicates an improvement in cardiovascular efficiency, allowing the heart to sustain levels of physical exertion with less effort after a nap. This further enhances efficiency, reflecting theories of recovery based on parasympathetic dominance and balance of the autonomic nervous system.

The study explores the physiological benefits of incorporating both short sleep stages (SWS and REM) into athletes' routines, particularly after partial sleep deprivation, to promote heart rate recovery and optimize performance outcomes. The 90 min nap is found to be particularly beneficial as it helps athletes manage their HRmax, which is crucial for coping with physical stress imposed by exercise. Shorter naps, such as 40- and 60 min naps, do not show significant improvements in HRmax, suggesting that SWS and REM stages may not have been fully engaged in autonomic processes for optimal heart rate recovery.

The study also found a marked decrease in Relative Pain Index (RPE) across nap durations, with the most significant decline observed after the 90 min nap. This suggests that athletes considered physical activities less tiring and stressful, which aligns with psychophysiological recovery theories. This effect could be related to deeper stages of sleep reached during a 90 min nap, namely SWS and REM sleep, which are essential for cognitive performance, mood stabilization, and mental fatigue reduction.

The lactate level findings agreed with the reducing values of RPE, as minimal increases in Lac levels, significantly lowered after the 90 min nap, mark anaerobic metabolic activity and indicate muscular fatigue. Lower lac concentrations after the nap indicate better metabolic clearance for recovery and, in this manner, reduce physical strain and thereby lower perceived exertion.

However, the inability to combine full SWS and REM cycle stages with shorter nap options explains the smaller absolute reductions observed in RPE amplitude, underlining the enhanced restorative effect of a full 90 min sleep. Longer naps are more effective in facilitating lactate clearance by including, besides SWS and REM stages, the deeper stages of sleep that are essential for ideal metabolic restoration and muscle recovery.

Reduced lactate levels following a nap support existing theories around metabolic recovery, suggesting improved oxygen uptake and replenishment of energy substrates, which in turn promotes more efficient lactate clearance. Longer naps that encompass both SWS and REM stages appear to facilitate greater metabolic restoration and mitigate muscle fatigue associated with lactate buildup. In contrast, shorter naps seem less effective in producing these benefits.

In conclusion, the study highlights the strategic value of integrating longer naps into athletes' routines, especially after partial sleep deprivation, to promote heart rate recovery and optimize performance outcomes.

Reduced lactate levels following a nap support existing theories around metabolic recovery, suggesting improved oxygen uptake and replenishment of energy substrates, which in turn promotes more efficient lactate clearance. Notably, longer naps that encompass both SWS and REM stages appear to facilitate greater metabolic restoration and mitigate muscle fatigue associated with lactate buildup. In contrast, shorter naps seem less effective in producing these benefits. SWS facilitates cellular repair and muscle glycogen replenishment, both critical to reducing lactate buildup and preventing muscle fatigue. In high-intensity physical performance, such as sprinting and jumping, lactate buildup impairs muscle function; thus, the ability of naps to support lactate clearance directly correlates with enhanced recovery and sustained athletic performance. Results have indicated that longer nap duration affords greater lactate clearance because both SWS and REM sleep stages are contained.1