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The purpose of this study was to investigate the effects of a 30-min nap, during a simulated night shift environment, when a prophylactic daytime sleep was implemented prior to the night shift. A repeated-measures counterbalanced design was used which included two experimental conditions: a 30-min nap and a no nap control. In both conditions subjects obtained a 2-h sleep in the afternoon from 15.00–17.00 hours, which was followed by the night-time nap from 02.30–03.00 hours in a controlled laboratory environment. Post-nap testing was conducted from 03.10 to 07.00 hours. The participants included 22 adults aged from 18–35 years who were good sleepers and did not regularly nap. Subjective alertness (Stanford Sleepiness Scale, Karolinska Sleepiness Scale, Visual Analog Scale), fatigue and vigor (Profile of Mood States), cognitive performance (psychomotor vigilance task, symbol–digit substitution task, letter cancellation task), and objective sleepiness were measured pre- and post-nap. The 30-min nap resulted in some impairment of subjective alertness for a brief period (up to 30 min) immediately following the nap when compared to the no nap condition. Following this brief period, alertness and performance were generally improved by the 30-min nap from 04.00 hours until the end of the testing period at 07.00 hours. The results of this study indicate that when a 2-h prophylactic sleep is implemented in the afternoon, a 30-min nap during the subsequent simulated night shift overall provides a significant countermeasure against sleepiness and performance impairment.
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The escalating trend in night shift work over recent years has resulted in the increased sleepiness1–4 and decreased cognitive and psychomotor abilities5,6 of employees in many workplaces. Consequently, the probability of serious human error and accidents in the workplace has also increased. Many researchers have suggested potential countermeasures against these detrimental effects, including napping during night shift work.7–14
Research has consistently demonstrated that a daytime nap results in increases in subjective and objective alertness and improved vigilance following the nap.3,15,16 Although the majority of research has focused on daytime napping, the alerting effects of a night-time nap has been found to be delayed after waking and last for only a short period of time.17,18
Naps of 60 min or longer taken during a night shift have been shown to improve subjective sleepiness and fatigue, relative to having no nap.10,18–21 A brief nap of 30 min or less has also demonstrated improved objective and subjective alertness, in addition to fatigue and vigilance.12,22 Generally longer naps have been demonstrated to be more recuperative than brief naps, with alerting benefits lasting for much longer after waking.23 Although the benefits of longer naps are more durable, the alerting benefits of brief naps are evident much sooner, within 10–15 min of waking.24 Individuals who have had a long nap usually do not report any improvements in their alertness within the first hour after waking, often feeling more sleepy and fatigued than they did before the nap.18–20
The increase in sleepiness experienced immediately after waking is referred to as sleep inertia. Although there are a number of factors which can contribute to sleep inertia,25,26 the three-process model of alertness proposes that the duration of sleep inertia is proportional to the amount of slow-wave sleep contained within a sleep episode.9,18,19,22 Longer naps, which contain a greater quantity of slow-wave sleep, result in a longer period of sleep inertia after waking.23 The amount of slow-wave sleep within a sleep episode is also proportional to the wake time prior to the sleep episode.23 Therefore, an individual who has been awake for many hours (e.g. 20 h) will experience more slow-wave sleep within their sleep period and more sleep inertia after waking. Long periods of sleep inertia can be dangerous within the workplace. Human error and accidents are more likely to occur during a period of sleep inertia, just as they are when an individual is very sleepy and fatigued prior to napping.9,23 Therefore, when considering napping as a countermeasure against sleepiness within the workplace, it is important to take precautions to minimize sleep inertia.
This study will assess the alerting benefits of a 30-min nap, compared to no nap, during a simulated night shift environment. A short nap has been chosen both to minimize the sleep inertia following the nap (relative to longer naps) and because it is more practical than a longer nap in most night shift circumstances. The three-process model of alertness predicts that by decreasing wake time (or sleep pressure) before the brief nap, less slow-wave sleep will be contained within the nap, consequently reducing sleep inertia.9 Therefore, this study also incorporated a long afternoon sleep prior to the night shift in order to reduce sleep inertia and allow improvements in alertness to be observed soon after the night shift nap. Research has shown that many shift workers take an afternoon nap prior to a night shift, in particular prior to a first night shift.27,28 To the knowledge of the authors, no other research has examined the alerting benefits of a night shift nap under the conditions of a prior afternoon sleep.
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The trends in alertness from baseline to each post-nap testing across nap conditions is shown for all dependent measures in Figure 1. When no nap is taken (dotted line), there is a steady decline in alertness from the baseline until the final testing time. Compared with this steady decline there was an immediate relatively sharp decline in subjective alertness following the 30-min nap. Table 3 shows the sleep architecture for this nap including the means (SD) for total sleep time, minutes of stage 1, stage 2, slow-wave sleep, and proportions of participants who woke from each sleep stage. It indicates considerable slow-wave sleep over the 30-min nap with 91% of awakenings from slow-wave sleep.
Figure 1. Mean changes in the (a) Stanford Sleepiness Scale, (b) Karolinska Sleepiness Scale, (c) Visual Analog Scale, (d) Profile of Mood States, fatigue subscale, (e) Profile of Mood States, vigor subscale, (f) symbol–digit substitution task, (g) letter cancellation task performance, (h) mean reaction time, (i) mean number of lapses, and (j) sleep latency (percentage change), from baseline across all post-nap testing times, following the 30-min nap condition (dotted line) and no nap condition (solid line). The asterisk indicates time-points at which the decrease in alertness in the 30-min nap condition was significantly different from the decrease of alertness in the no nap condition. Y-axes were reversed for the Stanford Sleepiness Scale, Karolinska Sleepiness Scale, Visual Analog Scale, fatigue, mean reaction time, and lapses.
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Table 3. Architecture of the 30-min night-time nap including mean total sleep time, minutes in stage 1, stage 2, and SWS in addition to the percentage of participants waking from each sleep stage
|Sleep parameter||30-min night nap|
|Stage 1||2.25 (1.72)|
|Stage 2||13.16 (6.68)|
|Sleep stage prior to waking|| |
| Stage 1||0|
| Stage 2||9%|
However, following this initial decline in subjective alertness, by about 70 min following the nap almost all dependant variables show better performance and alertness. Table 4 shows the results of two-way interactions between nap condition and testing time for all dependent measures. Significant interactions between nap condition and testing time were indicated for the SSS, KSS, VAS, and the POMS fatigue subscale scores, in addition to SDST performance, sleep latency, and the mean number of lapses during the PVT. The significant interactions indicate different trends in alertness over time as a result of the nap condition with there being less eventual decline in alertness following the 30-min nap.
Table 4. Two-way repeated measures anova interaction effects between condition (30-min nap vs no nap) and time (pre-nap vs post-nap testing times) for all dependent measures
|Subjective alertness||SSS||5.73, 120.3||5.37||<0.001***|
|POMS|| || || |
| Fatigue||4.29, 90.03||3.07||0.018**|
| Vigor||3.41, 71.63||2.18||0.089|
|Cognitive functioning||SDST||4.11, 86.35||6.78||<0.001***|
|PVT|| || || |
| Reaction time||2.24, 40.40||3.016||0.055|
| Lapses||3.83, 61.31||3.14||0.022**|
|Objective alertness||Sleep latency test||2.19, 43.73||4.36||0.016**|
Two-way interactions between the 30-min nap and no nap for the change from baseline to each testing time were conducted to disclose the source of the interactions in these measures. All significant interactions are indicated in Figure 1 with an asterisk. The SSS and the KSS measures of subjective alertness indicate significantly decreased alertness 10 min after taking a 30-min nap relative to no nap. However, following this immediate decrease, subjective alertness tends to recover following the 30-min nap producing less decrease than following no nap for almost all measures by 70 min post nap, significantly so for the VAS and POMS fatigue subscale. The sleep latency measure indicated less decrease of alertness at 75, 135, and 195 min following the 30-min nap when compared to no nap.
Two-way interactions for condition by time for cognitive performance measures indicate less impairment of cognitive performance from about 105 min following the 30-min nap. Relative to no nap, SDST performance is significantly better at 165 and 225 min following the 30-min nap. The mean number of lapses also increases less at 165 min following the 30-min nap when compared to no nap.
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The present study assessed the benefits of a 30-min nap, compared to no nap, during a simulated night shift environment, when a prior prophylactic daytime sleep was utilized. Without the 30-min nap all measures, including objective alertness, cognitive functioning, and subjective alertness indicated a general monotonic decline from baseline at 02.15 hours until the last testing at 07.00 hours. However, following the 30-min nap, apart from a brief decline in subjective alertness, the decline in alertness and performance for all measures was less marked following the 30-min nap than no nap. Some benefits from the 30-min nap are evident from as early as 75 min post nap and last up to 3 h after the nap for most measures.
The only indication of sleep inertia in the present study was a brief but greater decline in subjective alertness immediately following the 30-min nap. This result is consistent with previous findings18,22,24 which reported periods of reduced alertness immediately after waking from night-time naps of lengths comparable to the 30-min nap used in this study. In line with the three-process model of alertness, the amount of slow-wave sleep contained within the 30-min nap may account for the sleep inertia reported.9,19,22,36 The polysomnography data indicates that slow-wave sleep comprised slightly over 50% (mean = 17.84, SD = 6.87) of sleep, which is greater than that reported for previous studies which have examined the alerting effects of both a 30-min nap opportunity during night shift37 and a 30-min daytime nap.3,37 In the current study 91% of participants were woken from slow-wave sleep at then end of the 30-min nap. Research suggests that the sleep stage prior to waking can influence subsequent sleep inertia.25 Awakening during slow-wave sleep produces more sleep inertia than awakening in stage 1 or 2 sleep.25
However, the sleep inertia following the 30-min nap in this study was only significant for subjective measures of sleepiness and not for cognitive functioning and objective alertness. It is therefore not likely that the implementation of a 30-min nap during a night shift, when a prophylactic afternoon sleep is used, would leave workers at an increased risk of injury resulting from impaired performance or falling asleep during the immediate post-nap period.
The importance of developing an effective countermeasure to the sleepiness and fatigue experienced by night shift workers is highlighted by the overall decline in alertness, indicated in the no nap condition, throughout the testing period of this study. This decline in alertness across the testing period is the result of the combination of the increasing homeostatic sleep drive (process S) and the increasing circadian sleepiness (process C).38 Due to these processes, workers are at an extremely increased risk of sleepiness-induced accidents and injury particularly towards the end of their shift. This effect was ameliorated to some extent in all measures of alertness and performance by a 30-min nap at 02.30 hours.
Further research within this area should extend on these findings to compare the 30-min nap benefit when a prior afternoon sleep is not taken. It could be predicted that without a prior daytime sleep, as is common practice before the first night shift,27 the long wake period prior to the night-time nap would increase the amount of slow-wave sleep contained within the nap and consequently greater sleep inertia. The sleep inertia following the night-time nap may reduce the advantage of the 30-min nap over the no nap condition. Other nap lengths such as 10 min, which have shown immediate post-nap benefits when taken in the afternoon,3 could also be investigated within a simulated night shift environment with and without a 2-h prophylactic afternoon sleep. Future research should also consider whether the benefits demonstrated in this study would be maintained across consecutive night shifts when long daytime sleep periods are attempted. It would also be important to investigate whether these benefits are shown across different sorts of work, for example those requiring greater physical activity. The possibility that the results might be different for regular night shift workers could also be explored.
The 30-min nap (in combination with an afternoon nap to reduce prior sleepiness) has yielded promising results when taken during a first night shift. All measures used in this study have indicated greater alertness and cognitive functioning following the 30-min nap when compared to no nap. A brief period of sleep inertia followed the 30-min nap; however, this decline was only significant in a limited number of the subjective alertness measures used in the study. Based on the results of this study, the implementation of a 30-min nap during night shift could be used to maintain a safer work environment for workers and those around them.