Tomohide Kubo

Effects of the length and timing of nighttime naps on task performance and physiological function

OBJECTIVE To examine the effects of the length and timing of nighttime naps on performance and physiological functions, an experimental study was carried out under simulated night shift schedules. METHODS Six students were recruited for this study that was composed of 5 experiments. Each experiment involved 3 consecutive days with one night shift (22:00-8:00) followed by daytime sleep and night sleep. The experiments had 5 conditions in which the length and timing of naps were manipulated: 0:00-1:00 (E60), 0:00-2:00 (E120), 4:00-5:00 (L60), 4:00-6:00 (L120), and no nap (No-nap). During the night shifts, participants underwent performance tests. A questionnaire on subjective fatigue and a critical flicker fusion frequency test were administered after the performance tests. Heart rate variability and rectal temperature were recorded continuously during the experiments. Polysomnography was also recorded during the nap. RESULTS Sleep latency was shorter and sleep efficiency was higher in the nap in L60 and L120 than that in E60 and E120. Slow wave sleep in the naps in E120 and L120 was longer than that in E60 and L60. The mean reaction time in L60 became longer after the nap, and faster in E60 and E120. Earlier naps serve to counteract the decrement in performance and physiological functions during night shifts. Performance was somewhat improved by taking a 2-hour nap later in the shift, but deteriorated after a one-hour nap. CONCLUSIONS Naps in the latter half of the night shift were superior to earlier naps in terms of sleep quality. However performance declined after a 1-hour nap taken later in the night shift due to sleep inertia. This study suggests that appropriate timing of a short nap must be carefully considered, such as a 60-min nap during the night shift.

HOW DO THE TIMING AND LENGTH OF A NIGHT-SHIFT NAP AFFECT SLEEP INERTIA?

Napping is one strategy that may assist night shiftworkers to cope with sleepiness and fatigue. However, one potential disadvantage of napping is that awakening from naps is disturbed by sleep inertia, which has also been found to impair performance and/or mood, transiently. The authors examined the effects of the timing and length of a night-shift nap on sleep inertia in a laboratory setting. Twelve male university students (mean ± SD: 21.6 ± 2.8 yrs) participated in this 3-day experiment, during which included a simulated night shift (22:00–08:00 h) and subsequent day (11:30–17:30 h) and night sleep (00:00–07:00 h). The simulated night shift was designed to include one of five (four nap/one no-nap) conditions. The napping conditions differed by their timing and duration: 00:00–01:00 h (Early 60 min; E60), 00:00–02:00 h (Early 120 min; E120), 04:00–05:00 h (Late 60 min; L60), 04:00–06:00 h (Late 120 min; L120). Participants completed all the experimental conditions in a counterbalanced order. Rectal temperature (RT) was recorded throughout the simulated shift and polysomnography (PSG) was recorded during the nap period. Immediately before and after each nap, participants were required to complete a visual analogue scale (VAS) to assess sleepiness and a visual vigilance test (VVT). During the simulated night shift, a set of tasks (an English transcription task, a performance test battery, and a break) was repeated hourly, except during the periods of napping. For each nap condition, the VAS and VVT (reaction time [RT]; lapses >5 s) results were analyzed by two-way, repeated-measures analysis of variance (ANOVA) (nap [nap versus no-nap] × time point [pre-nap versus post-nap]). PSG and RT data were analyzed with one-way repeated-measures ANOVA. Marginally significant interactions were observed for RTs and lapses in VVT for the L60 nap condition (p = .071 and p = .070, respectively). However, those effect sizes were moderate (partial η2 = 0.266, 0.268, respectively). Post hoc analyses showed significantly longer RTs (p < .05) and more lapses (p < .05) following the L60 nap compared with no nap. In contrast, there was no significant difference in sleepiness between the L60, or any of the other nap conditions, and the no-nap condition. Our findings suggest the effect of sleep inertia on VVT performance was profound in the L60 condition, although no significant effects on sleepiness were self-reported by VAS. The dissociation between performance and sleepiness might reflect an unstable state where participants cannot perceive decline in their performance. The present findings are significant in terms of occupational safety; the practical implication is that great care is needed when taking a 1-h nap between 04:00 and 05:00 h on the night shift. (Author correspondence: kubo@h.jniosh.go.jp)

Weekend sleep intervention for workers with habitually short sleep periods.

OBJECTIVES This study was conducted to determine whether extended sleep time during the weekend improves alertness and performance during the subsequent week for workers who are habitually short on sleep time. METHODS Daytime employees in the manufacturing industry [38.3, standard deviation (SD) 8.1 years old, mean weekday sleep ≤6 hours] participated in a study that lasted 3 successive weeks. Participants were instructed to stay in bed for ≥8 hours between 22:00-09:00 hours on weekends during the first week as a sleep intervention condition and keep their habitual sleep-wake patterns as a habitual weekend sleep condition beginning the weekend of the second week through Thursday of the third week. Half the participants underwent the conditions in one order and the other half in the reverse. Sleep was monitored by an actigraph. A psychomotor vigilance task, subjective fatigue, and blood pressure were measured on Monday and Thursday during the afternoon each week. RESULTS Sleep duration on weekends was approximately 2 hours longer per day during the intervention. However, sleep duration during weekdays following the intervention returned to shorter periods. Significantly shorter reaction times and a smaller number of lapses on the psychomotor vigilance task were found on Mondays after the intervention than after the habitual weekend sleep. The opposite results, however, were observed on Thursdays. CONCLUSIONS Sleep extension on weekends may be effective in improving alertness and performance during the first days in subsequent weeks among workers with short sleep times. These benefits might be maintained if sufficient sleep duration continues.

Sleep, fatigue, recovery, and depression after change in work time control: a one-year follow-up study.

Objective: We examined how change in work time control was associated with sleep and health 1 year later. Methods: Work time control, sleep, fatigue, recovery, and depression were assessed at baseline (T1) and at follow-up (T2) for 2382 daytime workers. The change in work time control from T1 to T2 was classified into four groups: low to low, low to high, high to low, and high to high. Results: A repeated-measures analysis of covariance showed significant decreases in the frequency of insomnia symptoms and depressive symptoms from T1 to T2 for the low to high group, which were similar to the high to high group. Significantly lower fatigue was found for these two groups at T2. Conclusion: An increase in work time control, in addition to its stable high level, may produce beneficial effects upon sleep and health.

Effects of cumulative sleep restriction on self-perceptions while multitasking.

This study addressed a rarely studied question of self‐perceptions of performance and overall functional state during cumulative sleep restriction and the ensuing recovery period. Twenty healthy male volunteers, aged 19–29 years, were divided into a sleep restriction group (n = 13) and a control group (n = 7). On the first 2 nights, the sleep restriction group had an 8‐h sleep opportunity that was restricted to 4 h for the next 5 nights, and then restored to 8 h for the last 2 nights. The control group had an 8‐h sleep opportunity each night. Each day participants accomplished 50‐min multitask sessions and gave self‐ratings in their connection. Similar to our previous findings on multitasking performance, self‐perceived task performance, sleepiness and mental fatigue impaired during the sleep restriction and returned to baseline during the recovery phase. Self‐perceived mental effort, tension, task difficulty and task pace showed no sensitivity to the sleep restriction. We concluded that sleep‐restricted individuals can probably make use of some self‐perceptions when assessing their ‘fitness for duty’. However, at the individual level these measures seem to be inaccurate in revealing actual performance impairments.

Interactive Model of Subsidiary Behaviors, Work Performance and Autonomic Nerve Activity during Visual Display Terminal Work

Interactive Model of Subsidiary Behaviors, Work Performance and Autonomic Nerve Activity during Visual Display Terminal Work: Toshimasa Takanishi, et al. Department of Occupational and Environmental Health, Nagoya City University Graduate School of Medical Sciences

Exploring career anchors among occupational health nurses in Japan: A qualitative study

AIM This study examined the career anchor characteristics that are possessed by Japanese occupational health nurses. METHOD Sixteen occupational health nurses participated in the semistructured interview. Data analyses were conducted using descriptive qualitative methods. RESULTS The data showed the following five categories: practices concerning relationships and positions; development of occupational health practices; management skills for effective work; practices that are approved inside and outside the organization; and work and private life considerations. CONCLUSIONS This study described the career anchors among occupational health nurses in Japan. The participants emphasized the following: the importance of maintaining good cooperative relationships with workers and supervisors; balancing professional and organized labor; and practicing effective occupational health services. Moreover, the occupational health nurses emphasized receiving approval from inside and outside of the organization. These results were consistent with the actual practices of occupational health nursing.

Cross-sectional associations between daily rest periods during weekdays and psychological distress, non-restorative sleep, fatigue, and work performance among information technology workers.

A daily rest period (DRP; rest taken from daily work during a 24 h period), is essential to work recovery. This study examined DRPs’ distribution and association with health outcomes among information technology workers recruited from an internet panel (N=1,811). Participants completed a web questionnaire examining psychological distress as a primary outcome, along with non-restorative sleep, fatigue (stress reaction), and work performance. Logistic regression analysis showed elevated psychological distress when DRP was <12 h (OR: 2.54; 95% CI: 1.47–4.42) and <11 h (OR: 2.48, 95% CI: 1.17–5.26), although the 95% CI included 1 after adjusting for age, sex, and working and commuting hours. After the above adjustment, similar associations were found with non-restorative sleep and fatigue, but not work performance, when DRP was <12 h. These findings constitute the first analysis of a dose-response relationship between DRP and subjective health outcomes among white-collar workers.

Fatigue and Sleep Among Employees With Prospective Increase in Work Time Control: A 1-year Observational Study With Objective Assessment

Objectives: This observational study aimed to determine how 1-year changes in work time control (WTC) have an impact upon objectively measured fatigue and sleep among employees. Methods: Thirty-nine employees were divided into two groups according to whether or not their WTC increased from baseline to 1 year later. Psychomotor vigilance task (PVT) and wrist actigraphy were used to objectively measure fatigue and sleep, respectively. Self-reported outcomes were also measured. Results: The increased WTC group showed gradual improvements in PVT performance and sleep quality over the course of the follow-up period compared with the not-increased WTC group. Between-group differences were statistically significant for PVT lapses and tended to be significant for PVT speed after 1 year. Conclusions: A progressive increase in WTC could play a crucial role in reducing fatigue and promoting sleep among employees.

1222 Diagnosis of 1,561 compensated cases for overwork-related cerebrovascular/cardiovascular diseases (ccvds) known as ‘karoshi’ in japan, 2010–2014

Introduction Overwork-related health disorders including cerebrovascular/cardiovascular diseases (CCVDs) and mental disorders, known as ‘Karoshi’, have been the keen occupational health issue. Main criteria applied for compensation of overwork-related CCVDs in Japan were (1) extraordinary events, (2) excessive working hours in 1 week and (3) long working hours over 100 extra hours in 1 month or over average 80 extra hours in 2–6 months. The present study examined diagnosis of compensated cases as overwork-related CCVDs in Japan. Methods The investigation report for the occupational accident related to CCVDs prepared by 321 labour standards inspection offices all over Japan from January 2010 to March 2015 were collected to JNIOSH-RECORDS. Total 1561 cases which met these criteria were examined from the viewpoint of the diagnosis-related in focusing on sex, age, size of the industry. This study was supported by the research fund of MHLW (No. 150903–01). Results The majority (96%) of all compensated CCVDs cases was male. Fatal cases accounted for 39.2%. A mean age at the onset of CCVDs was 49.3 years old. Intracerebral haemorrhage (ICH) was the highest (28.6%) in the proportion of diagnosis related group, following subarachnoid haemorrhage (SAH, 18.5%), cerebral infarction (CI, 14.6%), hypertensive encephalopathy (HE, 0.3%) in a cerebrovascular disease group. In a cardiovascular disease group, myocardial infarction (MI, 17.1%) was the highest, following cardiac arrest (including sudden cardiac death) (CA, 14.3%), dissecting aortic aneurysm (DAA, 5.2%), angina pectoris (AP, 1.2%). By classifying the industry size, CCVD cases at the workplace hiring less than 50 workers was over half of all cases (52.2%). Discussion Focusing on cerebrovascular diseases accounting for 60% is useful for examining preventive measures. Characteristics of diagnosis contribute to discuss directions for preventing overwork-related CCVDs. It should be paid attention to preventive measures in small sized enterprises that occupational health services are difficult to reach.