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Journal of Occupational & Environmental Medicine:
doi: 10.1097/JOM.0b013e3182728ddb
Letters to the Editor

Predictive Ability of Sleep Duration for the Feeling of Being Refreshed by Sleep

Kawada, Tomoyuki MD, PhD

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Department of Hygiene and Public Health, Nippon Medical School, Bunkyo-Ku, Tokyo JAPAN

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Buxton et al1 examined the associations between sleep deficiency and physical symptoms, mainly in female patient care workers.

They concluded that sleep deficiency was significantly associated with perceived pain, functional limitation, and workplace interference. The same research team2 also reported the results of a cross-sectional survey targeting the same patient care workers that was conducted to evaluate the relationships among low back pain, inadequate physical activity, and sleep deficiency.

Sleep deficiency was significantly related to several factors in the work environment. In these reports, they operationally defined sleep deficiency as short sleep duration, sleep insufficiency, and/or the presence of insomnia symptoms in the preceding 4 weeks. Short sleep duration was defined as sleep less than 6 hours per day. Sleep insufficiency was defined as “never/rarely feeling rested on waking.” Difficulty in falling asleep, awakening in the middle of the night, and/or early morning awakening were used as three symptoms of insomnia. Several reports have indicated the existence of a significant relationship between sleep problems and handling of the working conditions during shiftwork. Krystal3 reviewed the effects of night, evening, or rotating shiftwork on the circadian rhythm and sleep/wake functions, including insomnia and excessive daytime sleepiness.

Walia et al4 also reported that rotating shiftworkers suffered from a significant delay of sleep onset (difficulty in falling asleep) but did not report disruptions of sleep maintenance (awakening in the middle of the night) or excessive daytime sleepiness.

In contrast, sleep maintenance was significantly disrupted in fixed night shiftworkers. We previously reported that rotating shiftwork schedules had no effect on sleep duration.5

Although sleep deficiency is precisely defined by Buxton et al1 as a general indicator of sleep problems, it is not yet a globally accepted indicator. The author previously reported that perceived sleep quality influenced the association between sleep duration and a depressive state and that sleep duration does not always change in parallel with sleep quality, as judged by a self-report feeling of being refreshed (rested feeling) after sleep.6

I want to present additional information to facilitate understanding of the global indicator of “sleep deficiency.” In other words, the predictive ability of several variables related to the physical status, mental status, and lifestyle factors, including sleep duration, for the loss of a feeling of being refreshed (rested feeling) by sleep was evaluated quantitatively to understand the content of sleep. This has also been called “sleep insufficiency.”

A total of 4648 male workers of a car-manufacturing company in 2011 were enrolled in this study. A self-administered questionnaire was used to record the history of tobacco smoking, alcohol intake, habitual exercise, sleep duration, and feeling of being refreshed by sleep. “Habitual exercise” was defined as “walking or equivalent exercise for more than one hour everyday.” Current history of receiving medications for diabetes mellitus (n = 126), hypertension (n = 318), and/or dyslipidemia (n = 178) was also used for the analysis. Waist circumference at the level of the umbilicus at the end of expiration was also measured. The 12-item version of the General Health Questionnaire (GHQ12)7 was administered to measure psychological well-being.

The author adopted a Likert-style scoring procedure on a four-point scale: “Better than usual,” “Same as usual,” “Worse than usual,” and “Much worse than usual” scored as 0, 1, 2, and 3, respectively; a higher score corresponded to poorer health. The maximum and minimum GHQ12 scores were 36 and 0, respectively. Among the 4648 workers, the numbers of day workers, two-shift workers, and three-shift workers and others were 2490, 1821, and 337, respectively. Informed consent was obtained from each of the study participants, and the study protocol was approved by the Ethics Committee. SPSS version 16.0J for Windows (SPSS Japan Inc., Tokyo, Japan) was used for the statistical analysis. Statistical significance was set at P < 0.05 in all tests.

The number of subjects who felt refreshed by sleep was 2916, and the number of subjects who failed to feel refreshed by sleep (“sleep insufficiency”) was 1731 (one missing value; Table 1). Logistic regression analysis identified age, GHQ12 score, lack of habitual exercise, short sleep duration, and two-shift (day and evening) work as being significant predictors of “sleep insufficiency.” The odds ratios and 95% confidence intervals (with the significance levels indicated within parentheses) of age, two-shift work against day work, and the GHQ12 score were 1.01 (1.00 to 1.02; P < 0.05), 1.4 (1.2 to 1.6; P < 0.001), and 1.1 (1.1 to 1.2; P < 0.001), respectively. The odds ratios and 95% confidence intervals (with the significance levels indicated within parentheses) of short sleep durations of less than 4 hours, 4 to 5 hours, and 5 to 6 hours against 6 or more hours were 22.3 (10.9 to 45.4; P < 0.001), 9.7 (7.8 to 12.1; P < 0.001), and 3.3 (2.8 to 3.9; P < 0.001), respectively (Table 2).

Table 1
Table 1
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Table 2
Table 2
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Our present results indicate that short sleep duration is the strongest contributor to a self-report of sleep insufficiency with a dose-response relationship. Other factors such as age, two-shift work (including night work), and the global health self-report score were also significantly related to sleep insufficiency. For maintaining good sleep, it is recommended that two-shift workers avoid short sleep duration and maintain psychological well-being. I also speculate that the working environment influences sleep insufficiency via physical and mental factors, although a definitive conclusion can be reached only by a longitudinal follow-up study.

Tomoyuki Kawada, MD, PhD

Department of Hygiene and Public Health

Nippon Medical School

Bunkyo-Ku, Tokyo JAPAN

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REFERENCES

1. Buxton OM, Hopcia K, Sembajwe G, et al. Relationship of sleep deficiency to perceived pain and functional limitations in hospital patient care workers. J Occup Environ Med. 2012;54:851–858.

2. Sorensen G, Stoddard AM, Stoffel S, et al. The role of the work context in multiple wellness outcomes for hospital patient care workers. J Occup Environ Med. 2011;53:899–910.

3. Krystal AD. How the circadian rhythm affects sleep, wakefulness, and overall health: background for understanding shift work disorder. J Clin Psychiatry. 2012;73:e05.

4. Walia HK, Hayes AL, Przepyszny KA, Karumanchi P, Patel SR. Clinical presentation of shift workers to a sleep clinic. Sleep Breath. 2012;16:543–547.

5. Kawada T, Shimizu T, Kuratomi Y, et al. Monitoring of the sleep patterns of shift workers in the automotive industry. Work. 2011;38:163–167.

6. Kawada T. Quality of sleep, sleep duration and depressive state. J Clin Hypertens (Greenwich). 2012;14:479.

7. Goldberg DP, Gater R, Sartorius N, et al. The validity of two versions of the GHQ in the WHO study of mental illness in general health care. Psychol Med. 1997;27:191–197.

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