Effects of a short afternoon nap on cognitive and psychomotor functions of non-sleep-deprived subjects

INTRODUCTION

Pilot fatigue, associated with circadian dysrhythmia, sleep loss, and long working hours, is a major flight safety concern.^[1,2] Napping is used as an effective countermeasure against fatigue in several safety-critical occupations.^[1,3,4] It is popular among civil and military aviators to maintain optimal performance when required to work beyond the usual duration.^{[1, 5,6]} Research showed that naps of 30-min or less are sufficient to reduce fatigue, improve performance, and lessen subjective sleepiness.^[1,3,7] However, longer naps are often associated with sleep inertia (SI), a period of physical and mental impairment immediately after awakening.^[3,8-12]

SI could be detrimental to aviators.^[1,2] Documented psychomotor and cognitive deficits due to SI include increased simple and complex reaction times, reduced grip strength, and impaired steadiness and coordination.^[3,9,12] It has been shown to lower scores in visual-perceptual tasks, memory, complex behaviors, and various cognitive tasks such as clock reversal, cancellation, and mental arithmetic.^[8,9] The magnitude and duration of SI’s impact depend on the complexity of the task performed, time from awakening to testing, sleep stage and depth, anxiety level, and circadian state.^[8,9,12] In the absence of significant sleep deprivation, the duration of SI was found not to exceed 30 min.^[1,3,9] However, SI may still adversely impact specific psychomotor and cognitive skills required in specialized professions with high mental workloads, such as aviation.^[9,11-13] Severity of SI could be greater at the circadian trough compared to other times of the day.^[1,8,9] Although SI significantly increases reaction time, reduces vigilance, and decreases the speed of information processing,^[1,11,13] the magnitude and duration of its adverse effects are not well quantified for the aviation context.^[3,8,9]

The possibility of sleep deprivation during flying duties is addressed by flight duty time limit regulations.^[1] In addition, many international airlines allowed short-duration (30–40 min) cockpit naps to address fatigue during long-duration flights.^[1] However, the effects of SI, especially following an afternoon nap, are a significant concern.^[3] Therefore, this study was carried out to estimate the aftereffects of a short 30-minute afternoon nap on cognitive and psychomotor functions of non-sleep-deprived subjects as relevant to the aviation setting.

MATERIAL AND METHODS

RESULTS

Fifty healthy male volunteers, aged 24–45 years (mean ± standard deviation: 33 ± 5.5 years), participated. All subjects had GSQS scores of <2, confirming adequate restful sleep the night before the test.^[14] The majority (78%, n = 39) of the participants had KSS scores of 3–5, while the remaining 22% (n = 11) had KSS scores of 1–2. This indicated that most subjects exhibited moderate levels of sleepiness^[14,15] typical of the afternoon circadian trough. None showed excessive levels of sleepiness before the tests.

The pre-nap data were not statistically different from morning recordings for all of the tests. As there was no evidence of circadian influence, the pre-nap data were considered as a baseline for analysis. All subjects fell asleep spontaneously within a few minutes of arriving at the sleep laboratory after the pre-nap recordings. The 30-min nap was monitored using EEG recordings and actigraphy, with the duration standardized by an alarm bell. The subjects began the post-nap tests immediately after awakening. EEG recordings were monitored throughout the 2-h post-nap period to rule out any recurrence of sleep.

Post-nap response time of SRTT was significantly higher (p < 0.0001) than baseline from the immediate post-nap to 2-h observation period. However, the frequency of correct clicks did not change (p = 0.0527) during the post-nap period [Tables 2 and 3]. It is inferred that motor speed was adversely affected by SI, while accuracy was unchanged.

In ST, the Stroop effect (measured as the difference in response time) did not change significantly (p = 0.379) after the nap [Table 4].

In DSST, response time was unchanged (p = 0.367) at 15^th-min post-nap, but it significantly decreased (p = 0.0088) from 30^th min onward. This lower response time was observed throughout the rest 2-h period. The accuracy of responses in the DSST did not change after the nap [Tables 5 and 6].

DISCUSSION

Available studies on SI had various conditions of sleep deprivation or sleep durations at night or during the day.^{[7-9,11,13,20,21]} However, relevant data on the effects of SI after “controlled rest” or “power naps” are limited.^[3,9,22] The effects of such naps on cognition and psychomotor aspects in adequately rested individuals need to be well understood.^{[6,7,11,12,23-25]}

In this study, none of the parameters differed significantly between the morning and pre-nap recordings, confirming that the parameters were not affected by circadian changes from morning to the pre-nap period (1330 h).

In SRTT, post-nap response time was significantly higher than pre-nap control and the effect was persisted for 2-h post-nap. However, there was no significant difference in the number of correct clicks after the nap. This indicated that the response speed was adversely affected due to SI; however, the accuracy was unaffected. The effect on response time was prolonged and remained for 2 h of the post-nap period. Similarly, Dinges et al. reported that about 60-min of rest/sleep in the afternoon resulted in significant performance decrement in simple reaction time and working memory-related tasks, which lasted for up to 35 min.^[8] It has been hypothesized (arousal hypothesis) that the performance decrement observed in SI, without concomitant sleep deprivation, could be primarily due to decreased levels of arousal leading to a general slowing down of cognitive processes without remarkable impairment in accuracy, irrespective of the difficulty of the task.^[5,13] Therefore, the effect on response time in SRTT could be purely based on delayed motor performance recovery, as stated by Ferrara and De Gennaro.^[25] A study on similar setting by Bhatt et al. showed that the dual task response time was prolonged immediately after 30 min of afternoon nap; however, the effect was transient and did not last till 21-min.^[19]

The ST was one of the most widely used tests in cognitive assessment of executive functioning such as working memory, planning, focused attention, and problem-solving.^[18] In literature, there are many varieties of STs and scoring systems, and they have been interpreted for cognitive, personality, and psychological facets. Here, the interference test scoring has been utilized, which was one of the most favored by many authors.^[26,27] In the present study, the change in the response time in the ST was not statistically significant. Salamé et al. reported that SI post-1-h nap led to cognitive impairment, such as delay in response time of logical reasoning and spatial memory tasks, without affecting accuracy.^[28] However, the study was done at night following a period of sleep deprivation.^[28] Speed of cognition, and not accuracy, was affected with different levels of impairment under conditions of SI.^[13,23,24] The lack of change in the Stroop effect in this study could be because the current paradigm may have produced only a relatively mild level of SI affecting the executive control functions.^[29]

The DSST gave an insight into higher-order cognitive functions such as visuospatial processing, attention, processing speed, and mental tracking.^[18,20] The current study showed that the response time of DSST did not change in the immediate post-nap period; however, the same was significantly faster after 30^th min of the post-nap period, and the improvement persisted for the rest of 2 h. This evidence showed that the response time of DSST was not adversely affected due to SI, and the subsequent improvement in response time could be due to the beneficial effect of the short nap.^[20] Moreover, the accuracy parameters were not affected by SI. These results were in disagreement with other studies wherein the SI adversely affected the capability of attention, vigilance, and tracking.^[23,30-32] This could be because of the difference in the setting as experimented in this study in comparison to the experimental protocol of previous studies. A NASA/FAA study in Boeing 747s where crew members were allowed 40 min of rest during the low workload phase of the flight showed consistent and positive improvement in performance and alertness till the end of the flight in comparison to their counterparts who were not allowed the in-flight rest opportunity.^[1,5,22]

Naps can counteract the effect of sleepiness by enhancing subjective and objective alertness, improving cognition, vigilance, and psychomotor ability.^{[1,4-7,9,19,22]} Ferrara et al. suggested that cognitive performance recovery was greater and more rapid than motor performance recovery.^[31] It could be possible that the cognitive recovery was rapid enough or not being affected at all by the level of SI induced by the short nap in this study.^[12,21] Similarly, the beneficial effect of a short nap in the early afternoon on higher-order cognitive functions and attention functions has been well documented in the literature.^{[10,12,22,24]}

There were certain limitations in methodology that could influence the outcome of this study. The subjects were non-aviators whose inherent cognitive and psychomotor skills could be distinctly different from aviators. Convenience sampling was used which reduces the strength of results. The nap was at a pre-scheduled time (1400 h) and was of fixed duration (30 min). It was not at a self-selected time or for a period as preferred by the subject. In practice, an individual is likely to take a nap when sleepiness becomes so intense that it interferes with ongoing activity and when opportunity or conditions arise. The cognition and psychomotor activity could be affected by circadian rhythm. Therefore, comparison with the same subjects or age and sex-matched controls would have been more objective to negate the effect of circadian influences.

CONCLUSION

The study was undertaken to estimate the effect of SI on aviation-critical mental functions following a short afternoon nap akin to the one allowed as per policy in civil aviation to mitigate pilot fatigue. The short nap of 30 min was sufficient to produce SI, slowing down psychomotor activities. However, the accuracy component of psychomotor functions and executive control functions was unaffected. In addition, higher-order cognitive functions became faster. Therefore, it is concluded that while SI following a short nap slows down motor responses but benefits higher-order cognitive functions such as visuospatial processing, complex mental processes, and attention.