Original ArticleNapping and the risk of type 2 diabetes: a population-based prospective study
Introduction
A growing number of epidemiological studies have linked sleep disturbances (insufficient sleep, fragmented sleep, circadian dysregulation, and obstructive sleep apnea) with adverse metabolic sequelae, including obesity, insulin resistance, and type 2 diabetes [1]. Also, short sleep (less than 5–6 h per night) and long sleep (8 h or more), as well as insomnia symptoms, are associated with a significantly increased risk of type 2 diabetes [2].
Napping is a common cross-cultural phenomenon occurring across the lifespan and increasing with advancing age [3]. There are, however, no widely accepted definitions or criteria of napping; it usually refers to brief periods of daytime sleep (lasting less than several hours). Naps have also been used as a countermeasure to sleepiness during night shifts at work. In a review of earlier napping literature, Dinges [4] concluded that three-quarters of adults reported napping and the usual duration was from about 30 to 90 min. Later, Pilcher et al. [5] showed in a one-week sleep-log study in a group of young and middle-aged adults that in almost half of them, the average nap lasted less than 20 min (often called power naps).
Napping has been regarded both as a health-promoting habit [3] and as a risk factor, or a proxy, for a variety of chronic conditions [6] and mortality [7]. Among the most notable of these adversities are obesity and metabolic syndrome [8]. Obesity is both a major risk factor for type 2 diabetes [9] and also increases the risk for sleep-disordered breathing, especially obstructive sleep apnea [10]. Furthermore, while excessive daytime sleepiness and need for naps can result from poor sleep, it has also been shown that a shared underlying factor, like chronic stress exposure, both promotes obesity and deteriorates sleep [11].
Contrary to the many studies on other aspects of sleep and metabolic changes, there are only few reports of the association between napping and diabetes [8], [12], [13], [14], [15], fasting hyperglycemia [16], glycated hemoglobin, or insulin resistance [17], respectively. These available studies suggest an association between napping and diabetes (mainly type 2 diabetes) or impaired glucose regulation. However, the direction of possible causation and the existence of known or unknown confounders have been inadequately explored.
In this study we assessed the association between napping and the risk of type 2 diabetes in a large, prospective, population-based cohort of Finnish adult twins, with 14-year follow-up data and detailed information on potential confounding or effect-modifying factors, and information on incident cases of diabetes from nationwide registers. We also examined the association among a subset of twins replying to a follow-up questionnaire. We wanted especially to address the role of obesity in this association.
Section snippets
Population sample
The Older Finnish Twin Cohort is a longitudinal study of Finnish twin pairs of the same gender born before 1958 with both co-twins alive in 1975. These pairs were selected from the Central Population Registry of Finland in 1974 [18]. Zygosity was determined using an accurate and validated questionnaire method [19]. The third questionnaire survey in 1990 was mailed to pairs born in 1930–1957 with both co-twins resident in Finland in 1987, and 16,179 twin individuals could be contacted (response
Results
The distribution of answers to the question on napping was as follows: no need, 34.3%; I would like to but I cannot sleep during the daytime, 15.7%; on two days weekly or more seldom, 35.2%; on three to five days weekly, 7.7%; and every or almost every day, 7.1%. There were no major differences in the distributions between age groups under 40, 40–50, and 50 years or older, respectively. There were no significant differences between the two genders. Details on these as well as distributions of
Discussion
Our results show that the risk for type 2 diabetes was significantly increased among middle-aged individuals who, as much as 14 years earlier, had reported napping daily or almost daily. The association remained essentially the same after adjustments for sleep-related covariates such as insomnia, snoring, or sleep length. However, the association became non-significant after adjusting for BMI, suggesting that napping is not an independent risk factor for type 2 diabetes. This conclusion was
Conflict of interest
The authors report no conflicts 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.2015.11.004.
Acknowledgement
This research was supported by the Academy of Finland (Grant nos. 263278, 265240, and 267882).
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