a Department of Nutrition, Harvard School of Public Health, Boston, MA 02115, USA.
b Proyecto Salud Coronaria, Institute of Health Research (INISA) University of Costa Rica, Costa Rica.
Reprint requests to: Hannia Campos, Department of Nutrition Room 353A, Building 2, Harvard School of Public Health, 665 Huntington Ave, Boston, MA 02115, USA. E-mail: hcampos{at}hsph.harvard.edu
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Abstract |
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Methods We studied 505 MI survivors and 522 randomly selected controls, matched for age, gender, and area of residence, in a population-based case-control study in Costa Rica. Participation rates were 97% for cases and 90% for controls. All subjects completed a physical activity questionnaire that included occupational and leisure time components with specific questions on siesta. Five siesta frequency categories (<1/wk, 14/wk, 56/wk, daily [1 h and <2 h], and daily [
2 h and <3:30 h]) were used to calculate the odds ratio (OR) by multiple logistic regression.
Results Compared to controls, cases were more likely to take daily siestas (44 versus 35%, P = 0.01), and spend more time per siesta (1:07 ± 0:04 versus 0:54 ± 0:04 h:min, P = 0.002). As compared to subjects with the lowest siesta frequency (<1/wk), the OR for MI among those in the highest category was 1.51 (95% CI : 1.022.25, P for trend = 0.006). After adjusting for risk factors, lifestyle, and health history the OR across the siesta categories were 1.0, 0.77, 1.28. 1.66, and 1.40 (P for trend = 0.02).
Conclusions Our data suggest that the practice of daily siesta is associated with increased risk of MI.
Keywords Physical activity, exercise, coronary heart disease, sleep, Latin America, Hispanic, trigger
Accepted 4 November 1999
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Introduction |
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Given the widespread prevalence of the siesta as a leisure time activity, it is surprising that the health implications of the siesta on coronary disease have rarely been directly addressed in epidemiological studies. While not necessarily defined as a siesta', involuntary napping, daytime sleepiness, or sleep attacks' have been described as a characteristic symptom of a variety of sleep disorders that include sleep apnoea syndrome, and are associated with cardiovascular disease morbidity and mortality.46 Consistent with this finding, participants in NHANES I, who reported that they had to take a nap often or almost always, were at increased risk of coronary disease, and more likely to be diabetic, overweight, hypertensive, and hypercholesterolaemic. The NHANES I result suggested that taking a siesta might not be beneficial.3
In the context of the siesta cultures, the siesta is predictive of mortality among the elderly in Israel.7 In contrast, a small case-control study in Greece suggested that the frequency and duration of siesta may protect from coronary disease.8,9 In these studies, the siesta was not associated with self-reported night sleep disorders.79
The mechanisms by which the siesta practice may increase coronary disease are unclear. The post siesta cardiovascular response very closely resembles the period soon after waking up in the morning, since both are characterized by increases in heart rate and blood pressure, which result in increased myocardial oxygen demand.10 This physiological response is probably responsible for the increased onset of acute myocardial infarction (MI) and other acute cardiovascular events including sudden death during the morning hours compared to the occurrence of these events at night.1113 Thus, it is reasonable to hypothesize that the siesta increases the exposure to a post sleep phenomena that may act as a trigger of MI.10 To address this issue we studied whether the siesta is associated with risk of MI in a large population-based case-control study in Costa Rica.
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Methods |
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Eligible case subjects were men and women who were diagnosed as survivors of an acute MI for the first time by two independent cardiologists at any of the three recruiting hospitals in the catchment area. In order to achieve 100% ascertainment, the three hospitals were visited daily by the study fieldworkers. All cases met the World Health Organization Criteria for MI, which require typical symptoms plus either elevations in cardiac-enzyme levels or diagnostic changes in the electrocardiogram.16 Cases were ineligible if they died during hospitalization, if they were 75 years at the day of their first MI, if they were physically or mentally unable to answer the questionnaire, or if they previously participated in the study as a control. Enrolment was carried out while cases were in the hospital's step-down-unit.
One population control for each MI case survivor, matched for age (±5 years), gender, and area of residence (county), was randomly selected using the national census information available at the National Census and Statistics Bureau of Costa Rica. Control subjects were ineligible if they have ever had an acute MI, or if they were physically or mentally unable to answer the questionnaire. The control subjects were visited at their house for recruitment.
Cases and controls who agreed to participate were visited at their homes for data collection. Visits were planned so that the interviews were carried out, on average, within 3 weeks of hospital discharge (for controls, hospital discharge of the corresponding case subject) and when possible, by the same interviewer. Identical questionnaires and data collection procedures were used for cases and controls. All subjects gave informed consent to the study approved by the Ethics Committee of the Harvard School of Public Health and the National Institute of Health Research (INISA) at the University of Costa Rica.
Data collection
Data collection included a general questionnaire, anthropometric measurements, and a blood sample. The questionnaire consisted of closed questions regarding socio-demographic characteristics, smoking, socioeconomic status, and medical history including personal history of diabetes and hypertension. Self-reported diabetes and hypertension were validated using the recommended definitions by the Expert Committee on the Diagnosis and Classification of Diabetes Mellitus,17 and the Third Joint National Committee on Detection, Evaluation, and Treatment of High Blood Pressure (JNCIII):18 fasting capillary whole blood glucose 110 mg/dl and/or taking glucose control medications; blood pressure
140 mmHg for systolic and/or
90 mmHg for diastolic, and/or taking anti-hypertension medications. The sensitivity, specificity, and predictive values positive (PVP) and negative (PVN) for controls were 80%, 97%, 75% and 98% for self-reported diabetes, and 52%, 96%, 93% and 70% for self-reported hypertension. Thus, the reliability of self-reported diabetes and hypertension in the Costa Rican population is high. It should be noted that a self-reported condition is an indicator of whether a subject is aware of a disease and thus more likely to have modified their lifestyle. When we control for these conditions, we used the self-reported criteria since we were most interested in adjusting for these possible changes in lifestyle. Because it is important to exclude subjects who might have changed their habits because of knowledge or suspicion of prior coronary disease, we evaluated prior diagnosis of angina pectoris using the Rose questionnaire.18 Physical activity was determined by asking subjects the average frequency and time spent on several occupational and leisure time activities during the last year, including specific questions on siesta. Siesta was defined as a sleep or rest period during the day, with a duration of less than 50% of the average major sleep period of an individual.1 These activities were grouped into six categories according to their intensity, or METS (metabolic equivalents). One MET is defined as the energy expenditure for sitting quietly or approximately 1 kcal per kg1 body weight per h1.19 The categories included: (1) Lying quietly in bed: afternoon siesta (nap or rest) and night sleep (0.9 METS); (2) Sitting (1.0 METS); (3) Light indoor activity (2.4 METS); (4) Moderate outdoor activity such as gardening, light agriculture and construction, and walking on flat surfaces (3.6 METS); (5) Vigorous aerobic activity such as heavy agriculture and construction, walking uphill, climbing stairs, jogging and other sports (7.1 METS), and; (6) Strenuous anaerobic activity such as carrying, pushing and lifting heavy objects (7.8 METS). Time spent on each activity including siesta was calculated as the product of frequency (times per wk) and duration (h per time). Energy expenditure for each activity was calculated as the product of frequency, time, and intensity (METS). This questionnaire was validated by its ability to predict fitness level measured by the Harvard Step test, plasma lipids, and obesity, in our previous studies in Puriscal, Costa Rica.20,21 Anthropometric measurements were collected by fieldworkers with subjects wearing light clothing and without shoes. All measurements were performed in duplicate and the average was used for analyses. Weight was measured on a Detecto bathroom scale that was biweekly calibrated. Body mass index (BMI) was calculated as the weight in kg divided by the square of the height in m. Blood pressure and a fasting blood sample for the lipid profile were always collected in the morning visit after subjects had voided urine and rested for 10 min. Blood pressure was taken in the seated position, in duplicate in the same arm by fieldworkers using mercury sphygmomanometers. Blood pressure and lipids among cases may have been altered by the infarction or treatment. Standardization for blood pressure measurements was based on multi-centre NHLBI DASH22 procedures. Blood samples were centrifuged at 2500 r.p.m. for 20 min at 4°C to separate plasma, and stored at 80°C until they were transported over dry ice to the Harvard School of Public Health for analysis. Plasma triglyceride, cholesterol and high density lipoprotein (HDL) cholesterol were assayed using enzymatic reagents (Boehringer-Mannheim, Indianapolis). Cholesterol determinations in our laboratory are standardized according to the programme for research laboratories specified by the Centers for Disease Control, and the National Heart, Lung, and Blood Institute.
Data analysis
Data analysis was performed using the Statistical Analysis Systems software (SAS, Cary, NC). Two separate fieldworkers in Costa Rica entered the data twice and error checked for coding and data entry errors. Since data were edited in Costa Rica, data entry errors, questions or inconsistencies were quickly corrected and/or verified. Crude means and frequencies for health characteristics and risk factors were compared using two-sided unpaired t-tests and the Mantel-Haenszel 2 test. Mean daily physical activity indicators and standard errors, adjusted for the matching variables age, gender, and area of residence were calculated using linear regression. Loge-transformations were used for the variables siesta, moderate outdoor, vigorous, and strenuous daily physical activities. Data on these variables are presented as geometric means plus or minus approximate standard errors. Multivariate analyses were carried out by multiple logistic regression with maximum likelihood estimation of the regression coefficients and their standard errors. Five siesta frequency categories (<1/wk, 14/wk, 56/wk, daily [
1 h and <2 h], and daily [
2 h and <3:30 h]) were used for these analyses. The relative risks were estimated as odds ratios (OR) for the lowest category of siesta frequency compared to the highest, based on the distribution among controls. Tests for trend were derived from logistic regression with a single term representing groups 15. This was carried out by assigning the median value of time spent on siesta to each siesta frequency category and modelling this value as a continuous variable. The variables included in each fitted model are indicated in the Tables.
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Results |
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Demographic characteristics and socioeconomic indicators differed significantly between cases and controls (Table 2). Compared to controls, cases were more likely to be retired or had occupations in the general services such as drivers, security officers, and other self-employed occupations (tailors, hairdressers, shoe repair, etc.). In contrast, controls were more likely to be professionals or hold labour intensive blue collar and agricultural jobs. Cases were also less educated, more likely to be divorced, separated or in common law relationships, less likely to attend religious services, and had a lower socioeconomic status than controls.
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Discussion |
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The association between increased siesta time and coronary disease is plausible because of the substantial evidence indicating that inactivity increases the risk of coronary disease.2325 Accordingly, NHANES I results showed an association between daytime somnolence and increased cardiovascular risk factors that mediate the effect of physical activity on coronary disease including high blood pressure and serum cholesterol levels, obesity, and diabetes.3 Consistently, we found that subjects taking longer daily siestas had increased prevalence of hypertension, diabetes, increased BMI and plasma triglycerides. Adjusting for these parameters decreased the OR among those taking long daily siestas (from 1.51 to 1.44). Interestingly, the OR among those taking shorter daily siestas, was increased from 1.30 to 1.55 after adjustment.
The association of an afternoon siesta with coronary disease could also be explained, in part, by the surge in heart rate and blood pressure observed during the period soon after the siesta, which closely resembles the period soon after waking up in the morning.10 The morning waking hours are characterized by an increased onset of acute MI and other acute cardiovascular events including sudden death when compared to the occurrence of such events at night.1113 Increases in heart rate and blood pressure, which result in increased myocardial oxygen demand, may act as triggers of these phenomena in the morning.10 Thus, the daily siesta, in addition to being part of a more sedentary lifestyle, may increase coronary disease risk by triggering cardiac events.
Although we cannot rule out the possibility that some underlying sleep-related diseases explained the need for a siesta among some subjects in this population, our data suggest that these potential causes are probably minor given our study results. The prevalence of severe daytime sleepiness (daily daytime sleep for at least one h) is 5.5% in England.26 Daytime somnolence two or more times per week is more prevalent among patients with obstructive sleep apnoea (19%), but this disorder would account for less than 4% of the general population.4 In our study at least 70% reported taking a siesta at least once per week, and 3544% reported taking daily siestas. Sleep disorders (either too short or too long night sleep) have also been associated with daytime somnolence and with increased risk of coronary disease, stroke, and mortality.3,27,28 However, we did not find a strong correlation between siesta and hours of night sleep (7:02 versus 7:12 h:min per day) in the highest compared to the lowest siesta frequency category, consistent with previous reports.8,9 Furthermore, a significant association between siesta and night sleep was still present when night sleep was adjusted for. The time spent on daily siestas in this report was obtained predominantly at the expense of less time spent on light indoor physical activities. Thus, our findings could not be explained solely on the basis of underlying sleep disorders among those who are more likely to have a siesta. Our data suggest that the daily siesta is a common leisure time sedentary behaviour that increases the risk of coronary disease in cultures where the siesta is tolerated. The choice of taking a siesta could be modified by physical activity promotion education programmes since it is well recognized that habitual moderate physical activity can significantly reduce the risk of coronary disease morbidity and mortality and improve longevity.2325
While we found an association between daily siestas and coronary disease, no association or a trend towards a beneficial effect was observed among those who reported having a siesta 14 times per week compared to those who reported taking a siesta less than once per week. This finding contrasts the study in Greece where they observed a protective effect of siesta on coronary disease among subjects who took daily afternoon siestas of 85 min. However, the Greek study did not control for physical activity and 63% of their population were engaged in manual labour compared to only 18% in the present study. It is possible that in Greece, those who are more physically active were also more likely to have a siesta. Short prophylactic naps can increase alertness and counteract the effects of sleep deprivation, but increasing the nap duration does not seem to further increase alertness.29,30 Thus, to take a short siesta 14 times per week may have a neutral effect on risk of coronary disease, and this habit could be beneficial because of positive effects on daily work performance.
As with many epidemiological studies of physical activity, inaccuracies in measurement and the cross-sectional case-control study design are a limitation of our study. Biased interviews could have occurred because interviewers could not be blinded as to the case-control status. Biased reporting could have occurred if cases were more likely to remember certain health behaviours that affect disease compared to controls, as studies on diet and coronary disease have shown.31 However, we have no reason to believe that the case subjects or interviewers would selectively recall or report more time spent on siestas as opposed to light physical activity, since none of these activities are considered coronary disease risk factors among the Costa Rican general population. We also did not find major differences in the total daily time reported for daily activities in cases versus controls (21:16 versus 21:46 h:min). In our study, interviewer bias was minimized by not informing the fieldworkers about the major hypothesis of the study. More importantly, the overall association between physical activity and coronary disease found in our study is consistent with the general consensus, thus validating the accuracy of siesta reporting.2325 Since random misclassification of physical activity could decrease the ability to detect associations with coronary disease, our findings on coronary disease, siesta and physical activity are striking. These findings could not be explained by confounding by other coronary disease risk factors, although we cannot exclude the possibility that some residual confounding may have persisted.
There have been no previous large epidemiological studies on the effects of siesta on acute coronary disease. As many as 250 000 deaths per year in the US are attributable to a lack of regular physical activity.32,33 Ethnic minority populations in the US are less active than Caucasians,15,34 and are more likely to originate from countries that are part of the siesta culture.2,15 Our results suggest that recommendations to substitute daily siesta time for more physically active leisure time activities may be worthwhile in preventing coronary disease, particularly amongst cultures were siesta is a socially acceptable behaviour.
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Acknowledgments |
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