To nap or not to nap: more questions than answers
We spend a third of our lives in sleep. One of the most common yet understudied sleep behaviours in human beings is daytime napping. While napping is traditionally viewed as a countermeasure to sleepiness and as a strategy to boost performance, especially in healthy younger adults or among shift workers, 1 the effects of napping in middle-aged to older-aged populations are largely unknown. One key question is whether naps are protective or harmful for heart health among older adults. A
... g Greek case-control study from the late 1980s 2 compared 97 men with an acute episode of coronary heart disease (CHD) and 90 control subjects, and showed a 30% reduction in the incidence of CHD associated with a 30 min afternoon nap. Over the past decade, growing epidemiological evidence has pointed to napping as a risk factor for cardiovascular disease (CVD), and a recent meta-analysis 3 summarised 11 prospective studies and concluded that there was a J-shaped dose-response relationship, in which the risk of CVD decreased with increasing nap duration until it reached 30 min/day, but started to increase with longer naps after the 30 min/ day threshold. While this meta-analysis suggests that napping duration could be an important factor to consider in studies of naps, most previous studies have only compared nappers and non-nappers, or have considered only the duration but not the frequency of napping. Furthermore, the majority of studies only examined CVD mortality, whereas non-fatal CVD events are much less studied in relation to napping. Many studies have failed to fully address the influence of potential confounders or the independent effects of napping. In their Heart paper, Hausler et al 4 examine the longitudinal association between self-reported napping frequency and average napping duration and incidence risk of both fatal and non-fatal CVD among 3462 men and women (mean age 57.1) with no history of CVD in the CoLaus study, a Swiss population-based prospective cohort. Over a median follow-up period of 5.3 years, 155 fatal and non-fatal CVD events developed. The authors found a 48% lower CVD risk (HR=0.52, 95% CI 0.28 to 0.95) for those who napped for one to two times per week compared with the non-nappers, both in the unadjusted and adjusted models (for age, sex, education, smoking, sedentary behaviour, body mass index, night-time sleep duration, and other primary CVD risk factors such as hypertension, diabetes and dyslipidaemia). However, the increased unadjusted HR (1.67, 95% CI 1.10 to 2.55) associated with a napping frequency of six to seven times per week was attenuated after adjustment for the above covariates (HR=0.89, 95% CI 0.