Introduction

Habitual daytime napping is a common practice worldwide.1 2 Whereas daytime napping has been consistently linked to overall mortality,1–3 the effect of napping on cardiovascular disease (CVD) remains unclear. A Greek cohort study found that habitual nappers have lower coronary mortality,4 and a Greek case control study found that nappers have a lower risk of coronary heart disease.5 In contrast, a case control study in Costa Rica,6 and three cohort studies in the USA,7 Japan8 and Germany,9 all reported increased risk either for cardiac events6 9 or CVD mortality.7 8 Other studies found no association between habitual daytime napping and CVD.1–3 10 Only a few studies considered nap frequency and investigated the effect of irregular napping on CVD. Whereas the Greek cohort study found that irregular nappers (taking naps once or twice per week) had lower coronary mortality,4 no relationship between irregular nappers (<5 naps weekly) and cardiovascular mortality was found in the German cohort study.9

Nap duration was reported to be important with respect to CVD. A meta-analysis found a J curved dose–response relationship between nap duration and CVD.11 They further reported that subjects taking long naps (≥1 hour) had a higher risk for CVD compared with subjects taking no naps, while no association between short naps (<1 hour) and CVD was found.11 Interestingly, a J curved relationship was also reported between nap duration and diabetes and metabolic syndrome.12 A large study (n=116 632) reported that, compared with non-nappers, the risk of CVD increased for subjects napping ≤1 hour and was highest for subjects napping >1 hour.13 In contrast, in the Greek cohort study, a lower risk for coronary mortality was found for subjects taking naps of any duration.4 Apart from nap duration, night time sleep duration in combination with napping seems to play a role with respect to CVD. A study showed that daytime napping is associated with an increased risk of major CVD events in subjects with >6 hours of night time sleep but not in subjects sleeping ≤6 hours/night.13 Contrarily, habitual nappers sleeping <6 hours/night were at higher risk of CVD mortality than habitual nappers sleeping 6–8 hours/night.7

Most studies only distinguished between nappers and non-nappers1–3 5–8 10 or investigated the effect of nap duration4 9 11 13 on CVD, but did not take the frequency of napping into account. Further, most studies only focused on CVD mortality, and failed to assess the effect of napping on non-fatal CVD events. Thus we aimed to investigate the effect of nap frequency and average daily nap duration over a week on fatal and non-fatal CVD events in a population based cohort.