Original ArticleNapping and associated factors: a Japanese nationwide general population survey
Introduction
Napping is a habit observed not only among infants, children, and older people, but also in other age groups [1]. Napping is influenced by culture [2], and in fact the custom of taking an afternoon nap (siesta) is well known in most Mediterranean and several Latin-American countries [3]. Although in Japan it does not seem to be common to have such a scheduled nap time, a global cross-sectional study of sleep habits has indicated that napping is relatively common in many industrial countries, including Japan [4].
Napping takes several forms [1], [5]: the first, known as replacement napping, provides compensation for nocturnal sleep loss; and the second, known as prophylactic napping, is preparation for nocturnal sleep loss. The third form, known as appetitive napping, is simply for enjoyment and relaxation, and does not affect the nocturnal sleep [6].
Previous studies conducted in laboratory settings have revealed the effects of napping on the human body in various age groups. Experimental studies on the effects of replacement napping in healthy young and middle-aged adults have demonstrated that a short period of napping (less than 30 min) helped participants to improve their subjective levels of sleepiness, fatigue, and mood, and was beneficial to task performance [1], [7], [8]. The effects of prophylactic napping on young adults are reported to last longer than those of caffeine [9]. Another study of both young and middle-aged adults found very similar napping patterns in both age groups, and similar effects on sleep quantity or quality [10].
Several experimental studies focusing on older people have revealed that the napping patterns in this age group differ from those of young and middle-aged adults [1], [11], [12], [13]. In comparison with young people, older people nap more frequently [12] and do so more in the evening [13]. Although recuperative effects of napping on mood and cognitive performance have been observed in older adults, individuals in this age group require more effort than younger individuals to achieve the same results in performance tests [11].
By contrast, persistent sleepiness immediately after awakening from a nap may reduce task performance and thinking ability. This is known as sleep inertia [14]. Several factors are known to affect sleep inertia, including prior sleep deprivation, sleep duration, and sleep stage prior to awakening [14], [15]. Although the duration of sleep inertia does not normally exceed 30 min [14], [16], care should be taken to avoid it because of its detrimental influence on work efficiency and attention [14], [15]. To avoid sleep inertia immediately after napping, a nap of less than 20–30 min is recommended to avoid the likelihood of deep non-rapid eye movement (NREM) sleep, or a nap of approximately 90 min to allow sufficient time for the body to undergo one NREM-REM cycle and to wake from REM sleep [1], [16].
Previous epidemiological studies have clarified the associations between napping and human health/disease. Previous cross-sectional studies have reported associations between napping and cardiovascular disease [17], [18], [19], [20], diabetes mellitus [21], [22], [23], hypertension [23], respiratory symptoms [21], pain [21], [24], [25], cancer [17], depression [17], [24], [26], high mental stress [23], cognitive decline [22], daytime sleepiness [10], [24], [25], [26], [27], and increased body mass index (BMI) [22], [25]. With regard to the association between napping and nocturnal sleep, a number of factors are reportedly associated with napping, including insomnia symptoms [24], [26], night-time fragmentation [21], [26], lower sleep efficiency [25], short night-time sleep duration [25], long sleep duration per day [22], [23], [26], [28], irregular sleep–wakefulness pattern [29], and use of sleep medication [30]. Several prospective studies have reported an association between napping and an increase in all-cause mortality [17], [18], [23], [28], [31]. These aforementioned studies have suggested that napping is a sleep habit that may have an important impact on human health.
However, not all studies have yielded similar results for the associations between napping and human health/disease. Some cross-sectional studies have found no association between napping and ischemic heart disease [28], congestive heart failure [28], hypertension [28], diabetes mellitus [28], stroke [28], cancer [28], or BMI [18], [28]. Furthermore, with regard to the association between napping and nocturnal sleep, some studies have found no associations with sleep problems [12], [22], [32], [33] and some other studies have found no associations with sleep duration [21], [22], [32]. Another prospective study found no association between napping and mortality risk [34]. Moreover, a follow-up study on the general population in Greece revealed that coronary mortality was decreased in nappers [3].
Such differences among epidemiological studies may be due to the use of different methods for evaluation for napping, thus limiting direct comparison of their results [35]. For instance, some epidemiological studies employed two-alternative questions about napping [23], [28], whereas others inquired about the frequency of napping and then established a cut-off point [12], [19], [24], [25], [27], [36], [37], [38], [39], or some inquired about napping duration and established a cut-off point [17], [18], [22], [31], [34]. Moreover, although age is considered to be an important factor for napping [1], many previous studies have investigated specific age groups only. This may have produced different results, considering that napping patterns are subject to age-related physiological changes. Therefore, further analyses using survey data from a wide range of age groups are necessary.
A limited number of epidemiological studies have been conducted on napping in Japan. Soldatos et al. [4] conducted a cross-national study on sleeping habits in ten countries and reported that the prevalence of habitual napping on weekdays in Japan was the lowest (12.0%) among those countries and that the median duration of napping was 60 min. In a cohort study of a group aged 40–79 years, Tanabe et al. [23] inquired about napping behavior using two-alternative questions and reported that 36.5% of men and 29.3% of women napped. In addition, the results of this longitudinal study indicated that napping was associated with an increased risk of mortality due to cardiovascular disease, non-cardiovascular/non-cancer internal diseases, and external causes [23]. However, those studies were conducted in limited community populations and not in the general population, and the data on napping collected were not satisfactory.
In the present study, therefore, we attempted to analyze data from a nationwide general population survey to investigate napping habits in Japan; we also conducted an exploratory analysis of the factors associated with high-frequency napping and long-duration napping. We believe that elucidation of napping behavior and its associated factors can be an important clue for the studies aimed to improve sleep habits in the future.
Section snippets
Study subjects and data collection
The present study was performed using data collected by the National Health and Nutrition Survey, which was conducted in November 2007 by the Japanese Ministry of Health, Labour and Welfare. The National Health and Nutrition Survey is a cross-sectional survey that is conducted annually and aims to ascertain the actual state of health, food intake, nutritional intake, and lifestyles of the Japanese people, and to obtain basic data for implementing effective measures for health promotion [40].
Results
After seeking permission from the Ministry of Health, Labour and Welfare, we performed statistical analysis of the anonymized dataset obtained from the National Health and Nutrition Survey. A total of 9611 individuals participated in at least one of the three parts of the National Health and Nutrition Survey, and the response rate was approximately 53.4%. Data from the following respondents were excluded from the analyses: (1) those under 20 years of age (N = 1792); (2) those who submitted
Discussion
This is the first study to have evaluated the napping habits of the Japanese general adult population using data from a nationwide epidemiological survey. Our results revealed that some of the factors associated with high-frequency napping and long-duration napping differ. In addition, high-frequency napping was associated with older age, whereas long-duration napping was associated with younger age. This feature has not been highlighted in previous epidemiological studies of sleep conducted in
Conflict of interest
The ICMJE Uniform Disclosure Form for Potential Conflicts of Interest associated with this article can be viewed by clicking on the following link: http://dx.doi.org/10.1016/j.sleep.2015.12.006.
Acknowledgements
This study was partly supported by Health Science Research Grants from the Ministry of Health, Labor and Welfare of the Japanese Government (H18-JUNKANKITOU-005, H20-JUNKANKITOU-IPPAN-002, and H25-JUNKANKITOU-IPPAN-007), and by a Research Grant from the Japan Society for Promoting Science and Technology Agency (18603012, 2006-2007, 22591301, 2010-2013, 26507012, 2014-2017). The authors report no other financial affiliation or relationship relevant to the subject of the article.
References (65)
- et al.
How do individuals sleep around the world? Results from a single-day survey in ten countries
Sleep Med
(2005) - et al.
Sleep inertia
Sleep Med Rev
(2000) The role of prescribed napping in sleep medicine
Sleep Med Rev
(2003)- et al.
Gender differences in nighttime sleep and daytime napping as predictors of mortality in older adults: the Rancho Bernardo study
Sleep Med
(2013) - et al.
Frequent napping is associated with excessive daytime sleepiness, depression, pain, and nocturia in older adults: findings from the National Sleep Foundation ‘2003 Sleep in America’ Poll
Am J Geriatr Psychiatry
(2007) - et al.
Habitual napping moderates motor performance improvements following a short daytime nap
Biol Psychol
(2006) - et al.
Coping strategies and their correlates with depression in the Japanese general population
Psychiatry Res
(2009) - et al.
Use of alcohol and hypnotic medication as aids to sleep among the Japanese general population
Sleep Med
(2007) - et al.
The role of sleep hygiene in promoting public health: a review of empirical evidence
Sleep Med Rev
(2015) - et al.
The effect of nap frequency on daytime sleep architecture
Physiol Behav
(2012)