New research presented at this year’s European Association for the Study of Diabetes (EASD) meeting in Munich, Germany (12-16 Sept) shows that daytime naps of are associated with a 45% increased risk of developing type 2 diabetes compared with no daytime napping.There appears to be no increased risk associated with naps shorter than 60 minutes. The study is by Dr Yamada Tomahide, University of Tokyo, Japan, and colleagues.
Sleep is an important component of a healthy life, along with a good diet and appropriate physical activity. Several recent studies have shown that a U-shaped curve describes the relation between the duration of sleep (hours of sleep/night) and metabolic diseases. However, some people cannot get enough sleep at night due to social life and work life related factors. Taking naps is widely prevalent around the world. Daytime naps are usually brief, but can range from a few minutes to a few hours. The frequency varies from taking an occasional nap to planned rest periods even several times daily for habitual nappers. Some individuals take a nap because they are excessively sleepy during the daytime as a result of a sleep disorder.
In this study, the authors performed a meta-analysis to investigate the association between napping and the risk of metabolic diseases, and to quantify the potential dose-response relationship. They searched electronic databases for articles published up to 2016. To ascertain the validity of the eligible studies, the quality of each report was appraised with reference to the STROBE statement. In addition, a tool called the Newcastle-Ottawa Scale for assessing the quality of observational studies in meta-analyses was used to assess the validity of each study.
The analysis included 307,237 Asian and Western across 21 reports. In each study, analyses were well adjusted for several confounders.Pooled analysis revealed that a longer nap (≥60 min/day) was associated with a 45% increased risk in developing type 2 diabetes compared with no napping. In contrast, a shorter nap (<60 min/day) did not appear to be associated with any increased risk. A dose-response meta-analysis showed a J-shaped relation between nap time and the risk of diabetes or metabolic syndrome, with no effect of napping up to about 40 minutes/day followed by a sharp increase in the risk at longer times, becoming clinically significant at 60 minutes onwards. However, analysis showed that nap time was not associated with an increased risk of obesity for either shorter or longer naps.
The authors conclude: “Longer nap was associated with increased risk of diabetes and metabolic syndrome. Further studies are needed to confirm the efficacy of a short nap.”
Regarding the potential reasons for the association, the authors note that short naps might have benefical effects on diabetes, but the mechanisms are still unclear. Daytime napping might be a consequence of night-time sleep disturbance such as obstructive sleep apnoea (OSA). Epidemiological studies have shown that OSA is independently linked to heart attacks, stroke, fatal and nonfatal cardiovascular events, and all-cause mortality.
OSA is characterised by recurrent episodes of apnoea and hypopnoea associated with repetitive episodes of intermittent hipoxemia (low blood oxygen), changes of intrathoracic pressure, and arousal. Intermittent hypoxemia promotes oxidative stress by increasing the production of reactive oxygen species in the blood and angiogénesis (formation of new blod vessels), through elevation of the blood pressure, and by promoting systemic and vascular inflammation with blood vessel dysfunction that contributes to chronic multiorgan morbidity/mortality related to cardiovascular disease and metabolic dysfunction,
Moreover, sleep deprivation has been linked to a decrease of leptin, an increase of ghrelin, and increased hunger and appetite; a harmful impact on carbohydrate metabolism, increased cortisol (stress hormone) secretion, and elevated sympathetic nervous activity; and low-grade inflammation.
A further point raised by the authors is that major depression is associated with an increased risk of diabetes. Moreover, we have to acknowledge the possibility of “reverse causality”, which means that persons with an increased risk (those who are sicker) are more likely to experience an outcome. The people taking long daytime naps could be more likely to be ill and have various risk factors for diabetes, morbidity or mortality.
The authors say: “Several studies have demonstrated beneficial effects of taking short naps less than 30 minutes in duration, which help to increase alertness and motor skills. A short nap finishes before the onset of deep slow-wave sleep. Entering deep slow-wave sleep and then failing to complete the normal sleep cycle can result in a phenomenon known as sleep inertia, in which a person feels groggy, disoriented, and even sleepier than before napping. Although the mechanisms by which a short nap might decrease the risk of diabetes are still unclear, such duration-dependent differences in the effects of sleep might partly explain our finding.”
“A short nap might have the effect of improving an abnormal circadian rhythm and modifying a variety of endocrine abnormalities caused by sleep deprivation.”