Effects of operational assessment of the 4:4 and 4:4/6:6 watch systems on sleepiness, fatigue, and stress responses during patrolling on a navy missile patrol boat

This is a self-archived version of an original article. This version may differ from the original in pagination and typographic details. Author(s): Title: Year: Version: Copyright: Rights: Rights url: Please cite the original version: CC BY-NC-ND 4.0 https://creativecommons.org/licenses/by-nc-nd/4.0/ Effects of operational assessment of the 4:4 and 4:4/6:6 watch systems on sleepiness, fatigue, and stress responses during patrolling on a navy missile patrol boat © 2022 the Authors Published version Myllylä, Mikko; Kyröläinen, Heikki; Ojanen, Tommi; Ruohola, Juha-Petri; Heinonen, Olli J.; Vahlberg, Tero; Parkkola, Kai I. Myllylä, M., Kyröläinen, H., Ojanen, T., Ruohola, J.-P., Heinonen, O. J., Vahlberg, T., & Parkkola, K. I. (2022). Effects of operational assessment of the 4:4 and 4:4/6:6 watch systems on sleepiness, fatigue, and stress responses during patrolling on a navy missile patrol boat. Chronobiology International, 39(9), 1233-1241. https://doi.org/10.1080/07420528.2022.2090374 2022 Full Terms & Conditions of access and use can be found at https://www.tandfonline.com/action/journalInformation?journalCode=icbi20 Chronobiology International The Journal of Biological and Medical Rhythm Research ISSN: (Print) (Online) Journal homepage: https://www.tandfonline.com/loi/icbi20 Effects of operational assessment of the 4:4 and 4:4/6:6 watch systems on sleepiness, fatigue, and stress responses during patrolling on a navy missile patrol boat Mikko Myllylä, Heikki Kyröläinen, Tommi Ojanen, Juha-Petri Ruohola, Olli J. Heinonen, Tero Vahlberg & Kai I. Parkkola To cite this article: Mikko Myllylä, Heikki Kyröläinen, Tommi Ojanen, Juha-Petri Ruohola, Olli J. Heinonen, Tero Vahlberg & Kai I. Parkkola (2022): Effects of operational assessment of the 4:4 and 4:4/6:6 watch systems on sleepiness, fatigue, and stress responses during patrolling on a navy missile patrol boat, Chronobiology International, DOI: 10.1080/07420528.2022.2090374 To link to this article: https://doi.org/10.1080/07420528.2022.2090374 © 2022 The Author(s). Published with license by Taylor & Francis Group, LLC. Published online: 28 Jun 2022. Submit your article to this journal View related articles View Crossmark data ORIGINAL ARTICLE Effects of operational assessment of the 4:4 and 4:4/6:6 watch systems on sleepiness, fatigue, and stress responses during patrolling on a navy missile patrol boat Mikko Myllylä a,b , Heikki Kyröläinen c,d , Tommi Ojanen e , Juha-Petri Ruohola f , Olli J. Heinonen g , Tero Vahlberg h , and Kai I. Parkkola d,i a Centre for Military Medicine, The Finnish Defence Forces, Turku, Finland; b Doctoral Programme in Clinical Research, University of Turku, Turku, Finland; c Neuromuscular Research Center, Faculty of Sport and Health Sciences, University of Jyväskylä, Jyväskylä, Finland; d Department of Leadership and Military Pedagogy, National Defence University, Helsinki, Finland; e Human Performance Division, Finnish Defence Research Agency, The Finnish Defence Forces, Tuusula, Finland; f The Navy Command Finland, The Finnish Defence Forces, Turku, Finland; g Paavo Nurmi Centre & Unit of Health and Physical Activity, University of Turku, Turku, Finland; h Department of Biostatistics, University of Turku, Turku, Finland; i Faculty of Medicine and Health Technology, Tampere University, Tampere, Finland ABSTRACT The operation of naval vessels involves watchkeeping 24 h per day, which is globally carried out by a variety of different watch systems. In this study, the rotating 4:4 and fixed 4:4/6:6 two-section watch systems were compared in terms of sleepiness, fatigue, and stress responses. The data collection took place on a Finnish Defence Forces’ (FDF) Navy missile patrol boat with 15 crew members serving as study participants. The data collection periods lasted two separate weeks (7 days, 6 nights) with the different watch systems. The subjective sleepiness of the participants was assessed before and after every watch using the Karolinska Sleepiness Scale (KSS). Stress responses were assessed daily by the recorded levels of salivary alpha-amylase (sAA), cortisol (sCor), immunoglobulin A (sIgA), and dehydroepiandrosterone (sDHEA). The participants’ sustained attention, inhibitory control, and working memory were assessed daily by cognitive tests (SART, N-Back). The heart rate variability (HRV) during an orthostatic test was used as an additional daily marker to assess the amount of psychological stress of the participants. In this study, the difference regarding sleepiness and fatigue between the study weeks was most visible in the subjective KSS, which clearly favored the 4:4/6:6 system. The results of sAA and sIgA also suggested that the subjects were psychologically less stressed during the study week with the 4:4/6:6 watch system. Cognitive test results (SART, N-Back) indicated that there were overall no significant differences in the subjects’ sustained attention, inhibitory control, or working memory during the study weeks or between the study weeks. The results of the HRV data during the daily orthostatic tests were inconclusive but there was some indication that the subjects were less stressed during the study week with the 4:4/6:6 watch system. In conclusion, the present study indicates that in navy surface operations: working with the fixed 4:4/6:6 watch system causes less sleepiness, fatigue, and psychological stress than working with the rotating 4:4 watch system. The study result is well in line with previous research regarding watch systems. ARTICLE HISTORY Received 14 February 2022 Revised 16 May 2022 Accepted 12 June 2022 KEYWORDS Shiftwork; watchkeeping; watch systems; sleep; sleepiness; fatigue; stress; navy Introduction In surface navy operations, continuous watchkeeping is needed to be able to operate 24 h per day. On the other hand, continuous watchkeeping together with sleepiness can lead to fatigue (Akerstedt 2007). Excessive fatigue has subsequently the potential to reduce vigilance and other aspects of cognitive performance (Goel et al. 2009). Fatigue is also a major factor in maritime safety because excessive fatigue is estimated to be associated with 25% of maritime accidents (Raby and McCallum 1997). For watch keeping in the navy, numerous watch systems are available and these require different numbers of watch sections. The number of watch sections refers basically to the number of working groups needed to maintain the watch keeping system. In a naval environment, there are usually two or three watch sections. Watch systems that consist of three watch sections are usually considered preferable in terms of sleepiness and fatigue than systems which consist of only two watch sections (Harma et al. 2008; Lutzhoft et al. 2010; Paul and Love 2021). Generally, the more watch sections a watch system has the more manning power is needed to sustain it. In terms of watch systems that consist of two watch sections, the 4:4/8:8 watch system has been considered most preferable regarding sleepiness and fatigue (Paul and Love 2021; Størkersen et al. 2011). The two sections 4:4/8:8 watch system consists of 4 CONTACT Mikko Myllylä [email protected] Turku FDF Health Centre, Centre for Military Medicine, The Finnish Defence Forces, P.O. Box 5, Turku FI-20241, Finland CHRONOBIOLOGY INTERNATIONAL https://doi.org/10.1080/07420528.2022.2090374 © 2022 The Author(s). Published with license by Taylor & Francis Group, LLC. This is an Open Access article distributed under the terms of the Creative Commons Attribution-NonCommercial-NoDerivatives License (http://creativecommons.org/licenses/by-ncnd/4.0/), which permits non-commercial re-use, distribution, and reproduction in any medium, provided the original work is properly cited, and is not altered, transformed, or built upon in any way. h daytime watches and rest periods and one 8 h nighttime watch and rest period. One 8 h rest period once in 24 hours seems to provide the opportunity for a sufficient daily continuous sleep period. Nevertheless, The twosection 4:4/8:8 watch system is not suitable for all conditions. Surface navy operations in the challenging, shallow, and rambling archipelago of Finland demand continuous high alertness. Therefore, 8 h work periods, especially, at night time and thus the two-section 4:4/8:8 watch system are generally considered unsuitable for naval operations along the Finnish coast. There is a lack of knowledge concerning other more suitable two-section watch systems for these conditions. Thus, the aim of this descriptive study was to compare the twosection 4:4 and 4:4/6:6 watch systems in terms of sleepiness, fatigue, and stress responses. Methods Participants Fifteen healthy FDF Navy soldiers (n = 12) and conscripts (n = 3) took part in the study. The participants were recruited and served in the same FDF Navy missile patrol boat where the data collection took place. They participated voluntarily and gave their informed consent for the study, and they did not receive any financial reward for their participation in the study. The participants were instructed to maintain regular sleep-wake schedules and avoid sleep deprivation for 3 days preceding the study weeks. All the participants also reported the approximation of the mean duration of their sleep on the last 3 days preceding the study weeks and the actual duration of sleep on the last day preceding the study weeks. A more detailed description of the study participants is shown in Table 1. The study is a part of a research project approved by the Ethical Committee of Tampere University Hospital (R18166/2018). Research permission was granted from the Finnish Defence Forces’ Defence Command (AO22851, 18.12.2018) Study design The data collection took place on a Finnish Defence Forces' (FDF) Navy missile patrol boat. The data collection periods lasted 2 weeks in total: the first week (7 days, 6 nights) with the 4:4 watch system and then another week (7 days, 6 nights) with the 4:4/6:6 watch system. During study weeks all data collection was conducted during the rest time of the crew. Same participants took part in both data collection periods, and there was a 16-day timespan between the study weeks to ensure that the participants had enough time to recover from the first data collection period. The studied 4:4 watch system was a rotating system containing two daily 2 h half watches at 16:00–18:00 h and 18:00–20:00 h. The studied 4:4/6:6 watch system was a fixed system containing the same two daily half watches at 16:00–18:00 h and 18:00–20:00 h. The main difference between the studied 4:4 and 4:4/6:6 systems in addition to the rotation were two daily 6 h watches at 20:00–02:00 h and 02:00–08:00 h in the 4:4/6:6 system. More detailed structures of the studied watch systems are shown in Figure 1. During the first study week (4:4 watch system), the vessel’s duty was to monitor and secure Finland’s territorial integrity. During the second study week (4:4/6:6 watch system), the vessel took part in a national naval exercise. In both weeks, standing watch duties contained a variety of tasks, which were essential to the operation of the vessel. However, each participant had the same tasks on both data collection periods. The standing watch duties did not require any significant physical effort. There were no major differences in wind conditions between the study weeks. During the first study week, the mean wind speed was 5.6 m/s (range 0–12 m/s) and during the second study week, it was 7.4 m/s (range 2–12 m/s). There were also no major differences in the sea state between the study weeks in the cover of dense archipelago. The Karolinska Sleepiness Scale (KSS) value was collected to the personal sleep diary of the participants before and after every watch to assess the subjective sleepiness. Once a day at 16:00 h or 18:00 h depending on the watch schedule, daily saliva samples were collected in the canteen of the vessel to assess the objective level of mental stress from salivary alfa-amylase (sAA), cortisol (sCor), immunoglobulin A (sIgA) and dehydroepiandrosterone (sDHEA). After collection of the saliva samples, an orthostatic test (5 min supine, 5 min standing) was performed in the cabin of the participants following the execution of cognitive tests (SART, N-Back). As an additional marker to assess the amount of mental stress, the heart rate variability (HRV) was recorded for 24 h on days 1–3 Table 1. Description of the study participants. n Range Mean Age (y) 15 19–45 29.1 Height (m) 14 1.65–1.87 1.80 Body mass (kg) 14 62.8–102.1 83.6 BMI (kg/m2) 14 19.8–32.0 26.0 Before study week with 4:4 watch system Sleep/day, last 3 days (h) 14 5.5–8.0 7.0 Sleep/day, last 1 day (h) 14 7.9–9.0 7.6 Before study week with 4:4/6:6 watch system Sleep/day, last 3 days (h) 14 6.0–8.75 7.5 Sleep/day, last 1 day (h) 14 5.0–10.0 7.0 2M. MYLLYLÄ ET AL. and 5–7. In addition, the HRV was recorded during the orthostatic test. The HRV measurement device had to be reloaded once during each study week and on both weeks, the reloading took place on the fourth day of the week. Tools The Karolinska sleepiness scale (KSS) The KSS is a valid subjective scale to assess sleepiness and may also represent as a feasible marker concerning fatigue (Akerstedt and Gillberg 1990; Akerstedt et al. 2004; Kaida et al. 2006; Lutzhoft et al. 2010). During the study weeks, the participants marked up a numeric KSS-value in individual sleep diaries as instructed. The scale consists of numeric values from 1 (extremely alert) to 9 (extremely sleepy–fighting sleep). Saliva samples In the current literature sAA, sCor, sIgA, and sDHEA are considered to be reasonable tools for assessing the amount of psychological stress (Strahler et al. 2010; Hellhammer et al. 2009; Afrisham et al. 2016; Izawa et al. 2008). In this study, saliva samples were collected daily using the Salivette® sampling device. The participants placed a swab in the mouth, chewing it for 60 s to stimulate salivation and returned the swab with absorbed saliva back to the Salivette® device. During the study weeks, saliva samples were contained in a freezer before further analysis. To obtain reliable results, the participants were instructed to avoid physical exercise for 3 h or eating or brushing their teeth for 1 h before taking the saliva sample. They also did not use any cortisol medication during these data collection periods. Figure 1. (a) Structures of the studied 4:4 watch system. (b) Structures of the studied 4:4/6:6 watch system. CHRONOBIOLOGY INTERNATIONAL 3 After thawing, the samples were centrifuged (Megafire 1.0 R Heraeus, DJB Lab Care, City of Germany, DJB Labcare Ltd, Buckinghamshire, UK) at 2000 × g for 10 min for the analysis. Saliva concentrations of cortisol (sCOR) and dehydroepiandrosterone (sDHEA) were analyzed with Siemens Immulite 2000 XPI (Siemens Healthcare, Malvern, PA, USA) using chemiluminescent enzyme immunoassay kits, while alpha-amylase (sAA), immunoglobulin A (sIgA) were analyzed by Konelab 20XTi (Thermo Fisher Scientific, Vantaa, Finland) using the enzyme photometric measurement method. The sensitivity and interassay coefficients of variance for these assays were 10.9 nmol/l and 6.02% for sCOR, 0.08 g/L and 4.35% for sDHEA, 4 U/L and 6.32% for sAA 5 μg·mL−1, and 1.2 pg/mL and 1.56% for sIgA. Orthostatic test An orthostatic test was performed daily because it was used as a measurement point for the heart rate variability (HRV) measurements. The execution of the orthostatic test was performed daily for both watch sections so that all subjects on the same watch section completed the test simultaneously. Instructions how to perform the orthostatic test were announced by the vessel’s on-board communication system. To obtain reliable results, the participants were instructed to avoid any other activity during the orthostatic test. Cognitive tests Cognitive tests (SART, N-Back) were conducted to assess the participants’ cognitive performance. The Sustained Attention to Response Task (SART) is a measure developed to evaluate sustained attention and inhibitory control (Robertson et al. 1997). The n-back task (N-Back) is a continuous performance task that is considered useful for experimental research in working memory and is also considered to predict interindividual differences in fluid intelligence (Jaeggi et al. 2010). During the study weeks, the participants performed the cognitive tests with laptop computers using the index finger of the dominant hand. To obtain reliable results, laptop computers were situated on tables, an unused new keyboard was attached to every computer and the participants performed the tests using the same keyboard and computer in both data collection periods. Completing the SART required approximately 5 min and the test consisted of numbers 1–9 appearing 225 times in random order. The participants attempted to respond to the appearance of each number by pressing a button on the keyboard, except when the number 3 appeared, which happened 25 times during the test. Completing the N-Back test required approximately 10 min and the test consisted of appearing letters. The participants attempted to respond to each letter that appeared instructed N items ago by pressing a button on the keyboard. In this study, the N-Back test was modified so that it consisted only of trials N = 0, N = 1 and N = 2. Trial N = 3 was left out of the N-Back in this study to shorten the duration of the test. Heart rate variability (HRV) HRV is characterized as the fluctuation of the length of hearth beat intervals and can be used to indirectly study the changes in the activity of the autonomic nervous system (ANS). Current neurobiological evidence supports the use of HRV as an objective marker to assess psychological stress (Kim et al. 2018). In this study, the HRV was recorded using the Bodyguard 2 device (Firstbeat Technologies Ltd., Jyväskylä, Finland) which recorded R to R waves measures at a sampling frequency of 1000 Hz. The HRV was assessed during a daily orthostatic test (5 min supine, 5 min standing). To obtain a steady signal and reliable timeand frequency-domain parameters, there was a 1-min baseline recording in supine and standing positions so that in both positions only the last 4 min of HRV were used for analysis (Bourdillon et al. 2017). This daily short-term (4 min supine + 4 min standing) HRV was analyzed using the Kubios HRV Standard program (version 3.4.3, Kubios Ltd., Kuopio, Finland). Three time-domain measures and five frequencydomain measures were utilized. The time-domain measures were the mean heart rate (HRmean) (bpm), standard deviation of NN intervals (SDNN) (ms), and the root mean square of successive RR interval differences (RMSSD) (ms). The frequency-domain measures were the absolute total power (TP) (ms2), absolute power of the very low-frequency band (VLF) (0.003–0.04 Hz) (ms2), absolute power of the low-frequency band (LF) (0.05–0.15 Hz) (ms2), absolute power of the highfrequency band (HF) (0.15–0.40 Hz) (ms2), and the ratio of LF to HF power (LF/HF). Statistical analysis A statistical analysis was performed using the SPSS statistical software (SPSS version 27.0.1.0; SPSS Inc., Chicago, IL). A Shapiro–Wilk test was used to assess the normal distribution of the data. Prior to the statistical analysis transformations were used for nonnormally distributed variables. Log-transformations were used with the saliva sample variables (sAA, sCor, sIgA, and sDHEA) and with some of the HRV variables (SDNN, RMSSD, TP, VLF, LF, HF, and LF/HF). A 1/ x-transformation was used with one SART variable 4M. MYLLYLÄ ET AL. (Mean RT) and an x2-transformation was used with another SART variable (Errors). Appropriate transformations were not found for the N-Back variables (Total hits, Errors) and a nonparametric test was used. Comparisons of normally distributed and transformed variables between the study weeks of 4:4 and 4:4/6:6 watch systems were performed using a linear mixed model to account for the repeated measurements of the participants. A linear mixed model was also used to evaluate the mean changes day by day from the first day of each watch system. To evaluate the baseline values at the beginning of the data collection periods, a paired samples T-test was used. Variables in the N-Back were analyzed using a nonparametric Wilcoxon signed rank test. P-values lower than 0.05 were considered statistically significant. Results The Karolinska sleepiness scale (KSS) The KSS was overall significantly lower during the second data collection period with the 4:4/6:6 watch system. There were statistically no significant differences between the first baseline values (1. day at 08:00) of the data collection periods regarding KSS. The mean KSS level concerning all data collection points was 4.2 during the 4:4 watch system and 3.7 during the 4:4/ 6:6 watch system (p < .001) (Figure 2). The highest KSS values were observed at night at 04:00 h or 02:00 h depending on the watch system. The mean KSS score for data collection points at 04:00 h vs. 02:00 h was 5.1 during the 4:4 watch system and 4.5 during the 4:4/6:6 watch system (p < .001) (Figure 2). There was also a significant difference in the KSS score at 16:00– 18:00 h when all other data collections were performed. The mean level of the KSS at 16:00–18:00 h was 3.8 during the 4:4 watch system and 3.4 during the 4:4/6:6 watch system (p = .046) (Figure 3). The daily mean level of the KSS at 16:00–18:00 h compared to the first day of the study week was significantly higher on the last 4 days during the 4:4 watch system and last 3 days during the 4:4/6:6 watch system (Figure 3). Saliva samples Results of the mean level of sAA, sCor, sIgA, and sDHEA during study weeks are shown in Table 2. The mean level of salivary alpha-amylase was significantly lower during the data collection period with the 4:4/6:6 watch system (p = .025). In contrast, the mean level of salivary IgA was significantly higher (p = .003), respectively. Between the study weeks, no significant differences in the mean levels of sCor or sDHEA were observed. Neither any statistically significant differences in the analyzed saliva biomarkers between the baseline values (1. day values) were noticed. Cognitive tests Concerning cognitive tests (SART, N-Back), there were statistically no significant differences between the study weeks. Neither were there any statistically significant differences between the baseline values (1. day values) of the data collection periods. Regarding SART, the mean reaction times for the correct response trials and Figure 2. The mean Karolinska sleepiness scale (KSS) level for all data collection points during the study weeks. Means and standard errors are shown. Comparisons were performed using a linear mixed model. ** Significant difference in the mean KSS level concerning all data collection points of the compared 4:4 and 4:4/6:6 watch systems. Arrows point out the high night-time levels of KSS (at 04.00 h during 4:4 system and at 02.00 h during 4:4/6:6 system). Missing values for all data collection points: 7.1% (4:4 system) and 9.4% (4:4/6:6 system). Missing values for night-time data collection points: 9.5% (4:4 system) and 4.8% (4:4/6:6 system). CHRONOBIOLOGY INTERNATIONAL 5 commission errors were assessed. In terms of the N-Back scores, the number of correct responses and commission errors were assessed. More detailed results of the cognitive test are displayed in Table 3. HRV during orthostatic test Results of HRV in supine and standing positions are shown in Table 4. From the time-domain measures, the mean HRmean was significantly lower in both supine and standing positions during the data collection period with the 4:4/6:6 watch system. However, in the supine and standing position, the HRmean differed statistically significantly between the baseline values (1. day Figure 3. The mean the Karolinska sleepiness scale (KSS) level concerning data collection points at 16.00–18.00 h during the study weeks. Means and standard errors are shown. Comparisons were performed using a linear mixed model. ** Significant difference in the mean KSS level concerning all data collection points at 16.00–18.00 h of the compared 4:4 and 4:4/6:6 watch systems. * Significant difference in the daily KSS level at 16.00–18.00 h compared to the result of first day of the study week. Missing values for all data collection points at 16.00–18.00 h: 1.9% (4:4 system) and 3.8% (4:4/6:6 system). Table 2. Mean (± SD), medians, and interquartile ranges (IQR) of salivary alpha-amylase (sAA), cortisol (sCor), immunoglobulin A (sIgA), and dehydroepiandosterone (sDHEA) during the study weeks. 4:4 watch system 4:4/6:6 watch system p-value Mean ± SD Median (IQR) Mean ± SD Median (IQR) sAA (U/ml) 106 ± 86 74 (89) 95 ± 74 73 (62) 0.025* sCor (mmol/l) 11 ± 10 8 (6) 11 ± 16 7 (6) 0.776 sIgA (mg/l) 92 ± 44 84 (40) 110 ± 81 83 (59) 0.003* sDHEA (ng/ml) 2.4 ± 2.3 1.3 (2.4) 3.2 ± 4.0 2.0 (3.1) 0.134 Prior to the statistical analyses a log-transformation was used with all variables due to a skewed distribution. Comparisons were made using a linear mixed model. * Significant difference in the mean level of the variable between 4:4 and 4:4/6:6 watch systems. Missing values concerning saliva samples: 1.0% (4:4 system) and 0.0% (4:4/6:6 system). Table 3. Mean (± SD), medians, and interquartile ranges (IQR) of the cognitive test results during the study weeks. 4:4 watch system 4:4/6:6 watch system p-value Mean ± SD Median (IQR) Mean ± SD Median (IQR) SART Mean RT (ms) 353 ± 108 321 (108) 358 ± 99 324 (94) 0.213 Errors (no.) 16 ± 7 18 (12) 16 ± 7 16 (9) 0.116 N-Back Total hits (no.) 28 ± 3 29 (2) 27 ± 5 29 (5) 0.233 Errors (no.) 1.6 ± 0.8 1.3 (1.0) 1.5 ± 1.0 1.1 (1.6) 0.582 For the Sustained Attention to Response Task (SART) the mean RT is defined as the mean reaction time in the correct response trials. Errors are defined as the no. of commission errors (in other words no-go trial responses). For the n-back task (N-Back) the total hits are defined as the no. of correct responses. Errors are defined as the no. of commission errors (in other words responses to nontargets). Prior to the statistical analyses in SART, a 1/x-transformation was used with the mean RT, and an x 2 -transformation was used with errors due to the skewed distribution. Comparisons regarding the SART variables were performed with a linear mixed model. Variables in the N-Back results were analyzed using a nonparametric Wilcoxon signed rank test. Missing values concerning SART were 1.0% (4:4 system) and 0.0% (4:4/6:6 system). Missing values concerning N-Back were 6.7% (4:4 system) and 3.8% (4:4/6:6 system). 6M. MYLLYLÄ ET AL. values). An analysis of other time-domain measures (SDNN, RMSSD) did not reveal any statistically significant differences between the study weeks. From the frequency-domain measures, the mean LF value was significantly higher in the standing position during the data collection period with the 4:4/6:6 watch system. There were no statistically significant differences in the baseline (1. day values) for the LF values in the standing position. However, the baseline (1. day value) for the LF values in the supine position was statistically significantly higher on the data collection period with the 4:4/6:6 watch system. An analysis of the mean values of other frequencydomain measures (TP, VLF, HF, and LF/HF) did not reveal any statistically significant differences between the study weeks. Discussion The 4:4/6:6 watch system enables a 2 h longer daily continuous sleeping period than the 4:4 watch system, which could be beneficial in terms of sleepiness, fatigue, and stress responses. The studied 4:4/6:6 system was also a fixed system and in a previous study rotating shifts have been found to be more fatiguing than fixed watch schedules (Arulanandam and Tsing 2009). Previous research points out that rotating watch systems can cause interrupted sleep and disturb physiological rhythms (Colquhoun 1985; Colquhoun et al. 1968; Shattuck and Matsangas 2016). In this study, the difference regarding sleepiness and fatigue between the study weeks was most visible in the subjective KSS, which clearly favored the 4:4/6:6 system. Previous studies regarding the effects of the time of watch have reported significant differences between night shifts (00:00–04:00 h/04:00–08:00 h) and day shifts in terms of sleepiness and fatigue (Colquhoun et al. 1987; Harma et al. 2008; Leung et al. 2006; Lutzhoft et al. 2010). Furthermore, in this study, the mean level of the subjects’ sleepiness was clearly higher during night shifts compared to day shifts according to the KSS results. However, there was a significant difference also at night (04:00 h vs. 02:00 h) in the mean level of the KSS results in favor of the 4:4/6:6 system. Especially during longer working periods these kinds of differences in sleepiness between the watch systems could have considerable operational significance. The present results of the sAA and sIgA tests suggest that the participants were psychologically less stressed during the data collection period with the 4:4/6:6 watch system. Previous literature points out that sAA increases Table 4. Mean (± SD), medians, and interquartile ranges (IQR) of the heart rate variability (HRV) in supine and standing positions during orthostatic tests in both study weeks. Supine 4:4 watch system 4:4/6:6 watch system p-valueMean ± SD Median (IQR) Mean ± SD Median (IQR) HRmean (bpm) 66 ± 11 66 (10) 64 ± 9 64 (12) 0.045* SDNN (ms) 53 ± 25 48 (24) 50 ± 21 44 (24) 0.406 RMSSD (ms) 57 ± 33 49 (32) 58 ± 33 49 (33) 0.573 TP (ms 2 ) 3102 ± 3974 2008 (2251) 2689 ± 2945 1558 (2026) 0.525 VLF (ms 2 ) 126 ± 125 95 (119) 117 ± 136 75 (94) 0.150 LF (ms 2 ) 1572 ± 2983 817 (1330) 1248 ± 1233 756 (1076) 0.998 HF (ms 2 ) 1374 ± 1711 828 (1324) 1322 ± 1846 644 (921) 0.690 LF/HF (%) 1.90 ± 1.75 1.33 (2.18) 1.79 ± 1.89 1.30 (1.35) 0.808 Standing 4:4 watch system 4:4/6:6 watch system Mean ± SD Median (IQR) Mean ± SD Median (IQR) p-value HRmean (bpm) 82 ± 9 82 (11) 78 ± 7 78 (8) < 0.001* SDNN (ms) 46 ± 18 43 (24) 45 ± 14 45 (18) 0.345 RMSSD (ms) 31 ± 18 26 (21) 30 ± 12 28 (16) 0.516 TP (ms 2 ) 2211 ± 1832 1928 (2082) 2122 ± 1378 1656 (1699) 0.101 VLF (ms 2 ) 155 ± 135 110 (180) 154 ± 125 135 (112) 0.357 LF (ms 2 ) 1673 ± 1613 1169 (1419) 1647 ± 1197 1327 (1328) 0.045* HF (ms 2 ) 383 ± 566 190 (411) 316 ± 334 186 (272) 0.610 LF/HF (%) 8.32 ± 7.33 6.53 (7.87) 9.16 ± 8.96 6.76 (8.54) 0.274 * Significant difference in the mean level of the variable between the 4:4 and 4:4/6:6 watch systems. Eight HRV measures were utilized. Three time-domain measures: mean heart rate (HRmean), standard deviation of NN intervals (SDNN) (ms), and root mean square of successive RR interval differences (RMSSD). Five frequency-domain measures: absolute total power (TP), absolute power of the very low-frequency band (VLF), absolute power of the low-frequency band (LF), absolute power of the high-frequency band (HF), and the ratio of LF to HF power (LF/HF). From the variables the HRmean was normally distributed. Prior to the statistical analyses a log-transformation was used with all other variables (SDNN, RMSSD, TP, VLF, LF, HF, LF/HF) due to a skewed distribution. Comparisons were performed using a mixed linear model. Missing values concerning HRV were 10.0% for the supine position and 11.1% for the standing position (4:4 system), 11.1% for the supine position and 11.1% for the standing position (4:4/6:6 system). CHRONOBIOLOGY INTERNATIONAL 7