Napping, Nighttime Sleep, and Cardiovascular Risk Factors in Mid-Life Adults

Epidemiologic studies consistently report increased risk of cardiovascular morbidity and mortality among persons who report short or long sleep duration.1 These studies have primarily focused on duration of nighttime sleep and have not independently considered potential benefits or risks associated with napping during the day. Napping may be beneficial, especially among short sleepers. Consistent with that interpretation are the results of a longitudinal study in Greece where frequent siesta takers had reduced cardiac mortality over a 6-year follow-up period.2 On the other hand, survivors of myocardial infarction reported taking more frequent and longer siestas, compared to healthy controls.3 In two longitudinal studies of elderly persons, all-cause, cancer, and cardiovascular disease mortality rates were higher among persons reporting longer daytime naps.4,5 The prevalence of napping reported in surveys and in diaries ranges from approximately 10% to 65%.6 People who report frequent napping tend to be older and report poorer health/functional status7–9 and more sleep problems,7,9 suggesting that napping may be a consequence of poor sleep and/or poor health. In a prospective study of elderly women, women who reported napping on a daily basis have elevated risk for cardiovascular and all-cause mortality over 7 years of follow-up.10 In another elderly sample, Goldman, et al., reported that taking a longer nap during the day was associated with shorter sleep the next night as determined by actigraphy.11 In contrast to the largely negative associations between habitual napping and general health, short-term laboratory studies generally report positive effects of a nap on neurobehavioral functions, such as alertness,12 cognitive performance,13 and memory.14 BRIEF SUMMARY Current Knowledge/Study Rationale: There is little information on ethnic differences in the relationship of napping behavior and nighttime sleep measured objectively. We conducted this study of actigraphy measured nighttime sleep and self-reported napping in an ethnically diverse sample of middle-aged men and women. Study Impact: The findings of the study identify the association of napping behavior and cardiovascular risk factors, and of napping on subsequent night's sleep. Differences in sleep duration related to napping may increase cardiovascular risk for certain ethnic groups. The purpose of our investigation was three-fold. First, we tested whether daytime napping was associated with objective and self-reported sleep characteristics measured by actigraphy, sleep diaries, and sleep questionnaires. Secondly, using actigraphy, we investigated the serial relationships between napping and nighttime sleep, i.e., whether daytime napping was associated with better or worse sleep the subsequent night and/or whether short or disturbed sleep was associated with napping the subsequent day. Thirdly, we tested whether frequent napping was associated with known cardiovascular risk factors, specifically obesity, elevated blood pressure, and physical inactivity. Finally, because the sample was multiethnic, and ethnicity is a correlate of both sleep and cardiovascular risk,15,16 we also explored whether the associations varied by race/ethnicity.