Daytime napping and ambulatory blood pressure monitoring: Relevancy in Asian populations

In hot climates, such as the Middle East, Africa, India, Southeast Asia, and Central and South America, it is common to discontinue strenuous physical activity at noon, when temperatures are highest, in favor of a midday nap, and resume work when the sun is lower, along with the temperature. This behavior is common to many species including the drosophila fly and may even have a biologic foundation.1 In affluent societies (frequently at higher altitudes), it is often thought that the daytime nap habit is a manifestation of laziness and inefficiency. In northern (usually affluent) and southern (sometimes poorer) societies, people may nap for different reasons, including fatigue, lack of sleep, or exhaustion in the former and the same reason and/or cultural habit in the latter. Since 24-hour ambulatory blood pressure (BP) monitoring is now being broadly advocated, daytime sleep may distort the elegant distinction between the daytime and the lower nighttime BP, commonly dubbed “nocturnal BP dipping.” Ignoring it, as is frequently done, misrepresents some commonly used variables such as daytime BP, nighttime BP, and circadian variations. Some may look at this as semantics; however, it is clear that nocturnal BP and the reduction from daytime BP (dipping), as well as BP variability influenced by sleep (among other important factors), may be the most important variables from a prognostic point of view.2-5 At the individual level, daytime sleep when included in the daytime average BP, may artificially lower the value and minimize the day-night difference as well as the day/night ratio.6-8 Similarly, another behavioral pattern that may distort the circadian BP recording pattern is nocturnal awakening, be it for urination (as commonly happens in older individuals), for nursing (as happens when having babies to care for), or other reasons.9, 10 When awake measurements are included in the nighttime average, they may artificially elevate it. When both daytime napping and nighttime awake periods are present, the dipping (or night/day ratio) will be artificially changed and mask the physiological normal ability for circadian variation of BP.11 Some relevant research has used diaries for defining sleep and awake periods, and others have used an objective measure actigraphy with similar results. A recent attempt to compare diaries, actigraphy, and polysomnography (the gold standard in evaluating sleep) found the agreement of self-report with polysomnography to be comparable to that of actigraphy.12 Beyond the individual level, both daytime napping13, 14 and nocturnal urination15, 16 have prognostic implications, particularly in the elderly and those with prevalent ischemic heart disease, that increases mortality. A meta-analysis of napping that evaluated 11 prospective studies involving 151 588 participants found that napping for 1 hour or more was associated with increased cardiovascular and all-cause mortality. Nocturia was also associated with increased mortality.15, 16 Possible explanations could include confounding such as silent atherosclerosis, age, employment, and ethnic origin. Another explanation may have a hemodynamic basis, as the rise of BP and to a lesser extent of heart rate after awakening, be it in the morning, daytime, or night, may explain the overall finding of the previously mentioned meta-aanalysis.17, 18 In this issue of the Journal of Clinical Hypertension, Hoshide and colleagues19 present a broad, relevant, and enlightening review in their article: “Role of ambulatory blood pressure monitoring for the management of hypertension in Asian populations.” Of course many important Asian populations including Indian, Thai, Indonesian, Philippine, and many others are not represented because there is an absence of relevant data. The authors discuss several important features specific to Asian (mostly Japanese and Chinese) populations: The authors downplay the role of daytime napping, suggesting that it is prevalent mostly in Asians of Chinese origin. Nevertheless, in the large JACC (Japan Collaborative Cohort Study for Evaluation of Cancer Risk) study27 of patients free of cardiovascular disease, it was prevalent in 36% of men and 27% of women of the 67 129 participants. In addition, in this Japanese study, there was an increased association of daytime napping with adverse outcomes, as in the previously mentioned meta-analysis. Certainly the issue of nocturnal BP and the dip from daytime BP are close and competing predictors of adverse outcome. One of the common arguments against the importance of dipping is its poor reproducibility. This is probably true, but it should be noted that the dip is dependent on both daytime and nighttime BP, both of which may be easily distorted by misclassification associated with daytime napping and nocturnal awakenings or both. However, in 100 patients with repeated monitoring, the reproducibility of dipping as a categorical variable was only 66% 28 At the same time, reproducibility of BP category (normal vs elevated) was similar at 70%. When 35 of these patients whose 24-hour systolic BP changed significantly (by >10 mm Hg) between the two monitoring sessions, the correlation of first and second 24-hour average was nil, whereas that of the dip (as a continuous variable) was significant (r=.45, P<.01). It should be acknowledged that long before nocturnal BP could be assessed, Sir Horace Smirk had defined a state of the “basal BP” as measurements taken in patients at rest who were completely relaxed and in the supine position and found that those were better at characterizing the individual then what he called “casual BP.”29 By today's standards, Smirk's “basal BP” is probably analogous to sleep BP. He and his colleagues found more than 60 years ago that basal BP and not the casual one was a significant predictor of outcome.30 The authors have no conflicts of interest to declare.