A brief pre-exercise nap may alleviate physical performance impairments induced by short-term sustained operations with partial sleep deprivation – A field-based study

ABSTRACT

The purpose of the study was to evaluate the recuperative efficacy of pre-exercise napping on physical capacity after military sustained operations (SUSOPS) with partial sleep deprivation. Before and after a 2-day SUSOPS, 61 cadets completed a battery of questionnaires, and performed a 2-min lunges trial and a 3,000-m running time-trial. After the completion of SUSOPS, subjects were randomized to either a control [without pre-exercise nap (CON); n = 32] or a nap [with a 30-min pre-exercise nap (NAP); n = 29] group. SUSOPS enhanced perceived sleepiness and degraded mood in both groups. Following SUSOPS, the repetitions of lunges, in the CON group, were reduced by ~ 2.3%, albeit the difference was not statistically significant (p = 0.62). In the NAP group, however, the repetitions of lunges were increased by ~ 7.1% (p = 0.01). SUSOPS impaired the 3,000-m running performance in the CON group (~ 2.3%; p = 0.02), but not in the NAP group (0.3%; p = 0.71). Present results indicate, therefore, that a relatively brief pre-exercise nap may mitigate physical performance impairments ensued by short-term SUSOPS.

KEYWORDS:

• Endurance
• fatigue
• military performance
• operational stress
• psychological strain
• sleep inertia

Introduction

Cognitive and physical performance is typically impaired during sustained operations (SUSOPS), comprising continuous and demanding work conducted for extended periods without adequate recovery (Haslam 1984; Guezennec et al. 1994; Nindl et al. 2002). During such multi-stressor tasks, sleep deficit constitutes a prime determinant of performance degradations (Lubin et al. 1976; Haslam 1982; Angus et al. 1992). Thus, implementation of short periods of sleep (i.e., naps) over a SUSOPS intervention has been regarded a potent behavioural strategy minimizing perceived sleepiness and fatigue, which, in turn, may extend operational effectiveness and partly counteract functional impairments (for reviews, see Naitoh et al. 1983; Naitoh and Angus 1987; Caldwell et al. 2009). Yet, although tactical nap intervals are often employed, whole-body endurance capacity is commonly compromised by SUSOPS (cf. Keramidas et al. 2018).

The efficacy of napping as a fatigue countermeasure is determined both by the nap duration and the post-nap interval. In particularly, following restricted nocturnal sleep, short daytime naps of ≤20 min seem to have a temporary restorative function shortly after awakening (Tietzel and Lack 2001; Hayashi et al. 2005; Brooks and Lack 2006). Longer naps (≥30 min), albeit more efficient in terms of recuperative duration (Lumley et al. 1986; Brooks and Lack 2006; Signal et al. 2012), entail an acute transient “sleep-inertia” phase, characterized by performance and alertness deficits (Tassi and Muzet 2000; Balkin et al. 2002). Provided that the “sleep-inertia” effects are dissipated (Ireland et al. 2002; Pointon and Marino 2013), there is evidence to suggest that, in sleep-deprived individuals, a relatively short (20–30 min) pre-exercise nap may alleviate reductions in cognitive, sprint and endurance performance (Waterhouse et al. 2007; Blanchfield et al. 2018). Yet, to our knowledge, the recuperative efficacy of pre-exercise napping on physical performance has not been evaluated following a period of multi-stressor tasks, involving sleep deficit, as commonly encountered during military SUSOPS.

We sought, therefore, to assess whether a 30-min pre-exercise nap would alleviate the physical performance impairments prompted by a 2-day military training with partial sleep deprivation. We anticipated that the pre-exercise nap would mitigate, at least to some degree, the detrimental effects of SUSOPS on endurance exercise capacity.

Materials and methods

Subjects

Sixty-one (54 males and seven females) healthy cadets of the Swedish Armed Forces, who were enrolled in the course “Applied Physical Training Theory for Warfare” of the school program of the Military Academy Karlberg, volunteered to participate in the study. Subjects were informed in detail about the experimental procedures, and gave their consent. The experimental protocol was approved by the Human Ethics Committee of Stockholm (2017/1:8), and conformed to the standards set by the Declaration of Helsinki. The study was part of a larger project examining the effects of short-term military SUSOPS on physical performance assessed by field (present study) and laboratory (see Keramidas et al. 2018) testing; cadets, however, took part in one of the two studies.

Study outline

Two days before and immediately after the end of SUSOPS, subjects completed a battery of questionnaires, and performed a lunges trial and a 3,000-m run; their sequence was always in the same order (Figure 1). Testing was conducted, in the morning hours, at the facilities of the Military Academy Karlberg (Stockholm, Sweden), and was supervised always by the same investigators. Subjects, who were familiar with the test procedure (cf. Malmberg 2011), performed the physical performance trials in small groups (10–15 subjects per group) that remained the same on all testing occasions. Before the pre-SUSOPS trials, subjects were requested to maintain their normal sleep and activity patterns. After the completion of SUSOPS, subjects were randomized to either a control [CON = 32 (29 males, three females); mean (range) age: 25 (21–38) years, height: 180 (158–191) cm, weight: 80.2 (58–110) kg], or a nap [NAP = 29 (25 males, four females); mean (range) age: 25 (21–37) years, height: 180 (158–195) cm, weight: 78.2 (52–97) kg] group. Approximately 2 h after the end of SUSOPS, the NAP group was allowed to nap for 30 min (cf. Waterhouse et al. 2007). All subjects reported that they were able to fall asleep during this period; the actual duration of the nap, however, was not recorded. During this period, the CON group was able to rest, but was not allowed to sleep. The post-SUSOPS trials were performed ~ 40 min after the end of the nap.

Figure 1. Schematic representation of the overall study protocol. POMS-SF: Profile of Mood States-Short Form, MFI: Multidimensional Fatigue Inventory, SSS: Stanford Sleepiness Scale.

Note: the Pre-trials were performed 2-days before SUSOPS.

The SUSOPS has been described in detail previously (see Keramidas et al. 2018). The SUSOPS, which took place at Berga (Muskö Naval Base, Sweden), commenced at 06:30 AM and finished 51 h later at 08:00 AM. During this period, all subjects followed the same schedule, and hence were exposed to similar operational stressors. Namely, they conducted almost continuous military-relevant tasks, requiring moderate physical and mental effort. Subjects, who most of the time were on their feet, were allowed to take short naps; the total sleeping time was estimated to be ~ 5 h (in day 1: from 16:00 to 18:00 PM, in day 2: from 03:00 to 05:00 AM and from 21:00 to 21:40 PM). They were allowed 3 meals day⁻¹ (~ 3,600 kcal day⁻¹), whereas water consumption was ad libitum. We did not perform any measurements of energy expenditure in the field; however, based on measures from previous SUSOPS studies, in which the energy expenditure typically ranged from 4,000 to 8,000 kcal· day⁻¹ (Tharion et al. 2005; Castellani et al. 2006; Tassone and Baker 2017), we assume that the subjects of the present SUSOPS were in energy deficit, to some degree.

Psychological measures

Ten minutes before the lunges trial, all subjects were requested to fill out the following questionnaires (hardcopy format), based on how they felt at that particular moment: (i) the Profile of Mood States-Short Form (POMS-SF; Shacham 1983), which is a 37-item self-evaluation questionnaire of six subscales: tension, depression, anger, vigor, fatigue and confusion. The description of subjects’ feelings was provided based on a five-point scale with anchors from 0-“not at all” to 4-“extremely”. (ii) The Multidimensional Fatigue Inventory (MFI; Smets et al. 1995), which is a 20-item self-rating multidimensional inventory measuring different aspects of fatigue: general fatigue, physical fatigue, reduced activity, reduced motivation and mental fatigue. Each subscale contains four items and the answer ranges from 1-“yes, that is true” to 5-“no, that is not true”. (iii) the Stanford Sleepiness Scale (Hoddes et al. 1973), which assesses the perception of sleepiness. The NAP group additionally completed the same questionnaires prior to the 30-min nap.

Lunges trial

Following a 5-min warm-up, subjects performed, in an indoor hall, as many alternating forward lunges as possible within a 2-min period. Each lunge started in an upright standing position; during the lunge, subjects were kneeling at ~ 90° forward, while the heel of the front leg was at least 10 cm in front of the back-leg knee that was placed ~ 5 cm above the floor. During the 2-min trial, a 3-sec break was permitted. During the trial, subjects wore underpants, combat shirt, combat jacket, combat trousers, socks, boots, body armor and combat vest, and carried a rifle and a 20-kg backpack. The validity and reliability of the lunges trial has been tested previously (cf. Malmberg 2011; Larsson et al. 2015).

3,000-m run

Twenty minutes after the lunges trial, subjects performed a self-paced 3,000-m running trial on an outdoor track (cf. Stray-Gundersen et al. 2001; Malmberg 2011). Subjects, who were dressed in sportswear, were instructed to run as fast as possible. Time was recorded for each subject to the nearest 0.1 sec. At the finishing line, subjects were asked to provide ratings for perceived exertion (RPE) using the 20-point scale ranging from 6-“nothing at all” to 20-“maximum”. The environmental conditions were similar in all trials. It was neither raining nor snowing; the mean (standard deviation) temperature, relative humidity and wind speed were 7.7 (3.1)°C, 85 (7)% and 2.1 (0.4) m· s⁻¹, respectively.

Statistical analyses

Normal distribution and homoscedasticity were tested with the Kolmogorov-Smirnov and Levene’s tests, respectively. A two-way analysis of variance (ANOVA) for repeated measures [group (CON group × NAP group) × time (pre-SUSOPS × post-SUSOPS)] was employed. As regards the psychological measures in the NAP group, the post-SUSOPS refers to recordings performed after the 30-min nap, unless otherwise stated. To assess whether the 30-min nap altered the psychological responses, a one-way repeated measures ANOVA [pre-SUSOPS × post-SUSOPS (before nap) × post-SUSOPS] was also performed. Mauchly’s test was conducted to assess the sphericity and, if necessary, the Greenhouse-Geiser ε correction was used to adjust the degrees of freedom. When ANOVAs revealed a significant interaction and/or main effect, pairwise comparisons were performed with Newman-Keuls post-hoc test. A Student’s t-test for unpaired samples was also employed to assess whether the changes (i.e., the difference between pre- and post-SUSOPS trials) in physical performance measures differed between groups. Cohen’s d effect sizes were also computed (values for d of ≤ 0.2, ≤ 0.5, and ≥ 0.8 are considered as small, moderate and large, respectively). Statistical analyses were performed using Statistica 8.0 (StatSoft, Tulsa, OK, USA). All data are presented as mean values with 95% confidence interval (CI), unless otherwise indicated. The α-level of significance was set a priori at 0.05.

Results

Psychological measures

SUSOPS increased self-reported sleepiness in both groups (p < 0.001), and especially (p < 0.001) in the NAP group [CON group: pre-SUSOPS = 2.6 (3.0, 2.2), post-SUSOPS = 4.3 (4.7, 3.9); NAP group: pre-SUSOPS = 2.7 (3.1, 2.4), post-SUSOPS (before nap) = 5.2 (5.6, 4.8), post-SUSOPS = 5.2 (5.6, 4.8)]; a response, however, that was not related to the 30-min nap (p = 0.87).

The mean values of POMS-SF subscales are summarized in Table 1. In both groups, SUSOPS impaired the perceived tension (p < 0.001), whereas anger and confusion remained unaltered (p > 0.05). Also, SUSOPS enhanced the subjective fatigue and impaired vigor (p < 0.001); which were more profound in the NAP group (p ≤ 0.001). The 30-min nap aggravated the SUSOP-induced drop in vigor (p < 0.001), and augmented depression (p = 0.03).

Table 1. Mean (95% confidence intervals) values of the Profile of Mood State-Short form (POMS-SF) subscales obtained prior to the exercise trials before and after a 2-day sustained operations (SUSOPS) without (CON group; n = 32) and with (NAP group; n = 29) a 30-min pre-exercise nap.

	CON group		NAP group
	Pre-SUSOPS	Post-SUSOPS	Pre-SUSOPS	Post-SUSOPS (before nap)	Post-SUSOPS
Tension	3.3 (4.2, 2.5)	1.8 (2.6, 1.1)†	2.2 (3.0, 1.5)	1.4 (1.9, 0.9)†	1.2 (1.7, 0.6)†
Depression	2.3 (3.3, 1.4)	1.3 (2.1, 0.4)	1.8 (2.9, 0.7)	2.5 (3.6, 1.5)	3.2 (4.3, 2.1)†*
Vigor	9.4 (10.6, 8.3)	5.7 (7.1, 4.4)†	11.0 (12.2, 9.7)	5.3 (6.8, 3.8)†	2.6 (3.5, 1.7)†‡*
Anger	3.3 (4.5, 2.0)	2.2 (3.1, 1.2)	2.6 (4.0, 1.1)	3.9 (5.7, 2.1)	3.4 (5.3, 1.6)
Confusion	1.8 (2.5, 1.1)	2.9 (3.9, 2.0)	2.0 (2.7, 1.3)	2.3 (3.1, 1.6)	1.9 (2.7, 1.1)
Fatigue	2.6 (3.5, 1.6)	6.6 (8.4, 4.9)†	4.0 (5.2, 2.9)	10.5 (11.9, 9.1)	10.5 (12.1, 9.0)†*

Note: In the NAP group, the post-SUSOPS refers to recordings performed after the 30-min nap.

Significantly different † from the pre-SUSOPS trial, ‡ from the post-SUSOPS (before nap) trial, and * from the CON group (p ≤ 0.05).

The mean values of MFI subscales are presented in Figure 2. SUSOPS enhanced the pre-exercise feelings of general, physical and mental fatigue, and of reduced activation and motivation (p < 0.001). The increase in reduced motivation was greater in the NAP group (p < 0.001). None of the MFI subscales was altered by the 30-min nap (p > 0.05).

Figure 2. Mean (95% confidence intervals) values of the Multidimensional Fatigue Inventory (MFI) subscales obtained prior to the exercise trials before and after a 2-day sustained operations (SUSOPS) without (CON group; n = 32) and with (NAP group; n = 29) a 30-min pre-exercise nap.

Note: In the nap group, the post-susops refers to recordings performed after the 30-min nap. Significantly different † from the pre-SUSOPS trial, and * from the CON group (p ≤ 0.001).

Physical performance measures

During the post-SUSOPS lunges trial, the number of repetitions was decreased by ~ 2.3% in the CON group, albeit the difference was not statistically significant (p = 0.62, d = 0.13; Figure 3A). In the NAP group, however, the number of repetitions was increased by ~ 7.1% (p = 0.01, d = 0.35, Figure 3A). No differences were observed between groups at any time point (p > 0.05). The mean change in lunges performance, however, differed between groups (p = 0.01; d = 0.62).

Figure 3. Mean (95% confidence intervals) number of repetitions during the lunges trial (A), and endurance time during the 3,000-m running trial (B) performed before and after a 2-day sustained operations (SUSOPS) without (CON group; n = 32) and with (NAP group; n = 29) a 30-min pre-exercise nap. Significantly different † from the pre-SUSOPS trial (p ≤ 0.05).

During the 3,000-m trial, SUSOPS impaired performance by ~ 2.3% in the CON group (p = 0.02; d = 0.23), whereas it remained unaltered in the NAP group (p = 0.71, d = 0.03; Figure 3B). No differences were observed between groups at any time point (p > 0.05). Yet, the mean change in running performance tended to be greater in the CON than in the NAP group (p = 0.08; d = 0.38). The final values of RPE did not vary either between or within groups [CON group: pre-SUSOPS = 18 (0), post-SUSOPS = 18 (1); NAP group: pre-SUSOPS = 18 (0), post-SUSOPS = 18 (0); p > 0.05].

Discussion

In line with previous studies (Guezennec et al. 1994; Nindl et al. 2002; Lieberman et al. 2005; Keramidas et al. 2018), the short-term, multi-stressor military training degraded mood, and enhanced perceived sleepiness and subjective sensations of fatigue. The 2-day SUSOPS also compromised whole-body exercise performance, mainly during the aerobic endurance task (i.e., running time trial), and slightly during the muscular/strength endurance task (i.e., lunges trial). The 30-min pre-exercise nap, however, seemed to counteract, to some extent, the SUSOPS-induced reduction in physical capacity during both trials. Present results therefore indicated that, despite the complex nature of SUSOPS-mediated fatigue (cf. Henning et al. 2011; Keramidas et al. 2018), a relatively brief, pre-exercise nap may constitute an efficacious operational strategy alleviating the SUSOPS-driven impairment in exercise endurance.

The mechanisms underlying the beneficial effects of pre-exercise napping are unclear. It is noteworthy, however, that the SUSOPS-engendered negative affects prevailed, and were even somewhat aggravated (i.e., lower self-reported vigor and motivation, and higher depression), after the 30-min nap interval. Although negative affectivity and mental fatigue may influence, to a large extent, whole-body endurance performance (cf. Wilmore 1968; Marcora et al. 2009; MacMahon et al. 2014; Wagstaff 2014), present results imply that the nap-related performance restorations were independent of the individuals’ pre-exercise psychological state. Presumably, the unpleasant emotions post-nap reflected the transient “sleep inertia” effects of napping, characterized by performance and alertness deficits (Tassi and Muzet 2000), typically peaking shortly after awakening (Jewett et al. 1999). In the current study, the state of emotional perturbations was prominent ~ 40 min after the nap completion; a response that was most likely associated with the 30-min nap duration. Thus, previous studies have demonstrated that, following nocturnal sleep restriction, the adverse effects of “sleep inertia” incurred by a 30-min daytime nap, contrary to shorter naps (i.e., 10 min), were accentuated largely within the first hour after awakening, and were fully reversed 95 to 155 min thereafter (Tietzel and Lack 2001; Brooks and Lack 2006). There is also evidence to suggest that the attainment of deep sleep stages may potentiate the length and severity of post-nap “sleep inertia” (Brooks and Lack 2006); the sleep architecture of the nap, however, was not assessed in our study.

To minimize bias on subjects’ perception of the potential benefits of napping, the objectives of the study were not made explicit to the subjects. Yet, considering that both groups were unblinded to the interventions, whether, or to what extent, the post-SUSOPS performance variations were confounded by either a placebo effect in the NAP group and/or a nocebo effect in the CON group remain uncertain (cf. Beedie and Foad 2009). Nevertheless, the pre-exercise psychological state did not differ between groups, and the 30-min nap, did in fact, aggravate several of the SUSOPS-mediated negative affects, viz. in perceived vigor, depression and intrinsic motivation.

In conclusion, present findings indicate that a relatively brief pre-exercise nap may mitigate physical performance reductions ensuing brief, multi-stressor military training with partial sleep deprivation. Further investigation is required to identify the mechanisms underpinning the apparent recuperative benefits of pre-exercise napping.

Declaration of interest

The authors state that there is no personal or financial conflict of interest in the present study.

Acknowledgments

We are grateful to all subjects for their voluntary participation in the study.

Additional information

Funding

The study was funded by the Royal Institute of Technology-KTH, and the Swedish Defence University, both in Stockholm, Sweden.