Participants. Participants were 116 men and 100 women who took part in the Whitehall II psychobiology study. The Whitehall II cohort is a sample of 10,308 London-based civil servants recruited in 1985-1988 when 35-55 years old to investigate demographic, psychosocial, and biological risk factors for coronary heart disease (25). The psychobiology study involved 228 volunteers (123 men and 105 women) who underwent laboratory stress testing, 227 of whom carried out ambulatory blood pressure and heart rate monitoring over a working day (26, 27). Participants were of white European origin, 45-59 years old, living in the London area, not planning to retire for at least 3 years, no history or objective signs of coronary heart disease, and with no previous diagnosis or treatment for hypertension. All women were peri- or postmenopausal. Grade of employment was used as the marker of socioeconomic status, and participants were systematically recruited from higher, intermediate, and lower grades. The study was approved by the University College London/University College London Hospital's Committee on the Ethics of Human Research.
Procedures. Blood pressure and heart rate monitoring. Blood pressure and heart rate monitoring was carried out by using the SpaceLabs (Redmond, WA) 90217 monitor, an instrument that satisfies international instrumentation protocols (28). Ambulatory monitoring was carried out during a working day, beginning from 0730 to 0930 hours (depending on the work schedules of the participants) and continuing until bedtime. Readings were automatically triggered every 20 min, and the participant was instructed to keep still during cuff deflation. Blood pressure and heart rate readings were stored and not available to the participant at the time. Each reading was accompanied by an entry in a diary in which the participant recorded location and activity during the past 5 min.
Salivary cortisol measurement. Eight saliva samples were collected over the same working day and eight on a leisure day (Saturday or Sunday). Samples were taken at 2-h intervals from 0800-0830 to 2200-2230 hours. Tubes were returned to the investigators personally or by mail, and cortisol was analyzed by immunoassay at the University of Düsseldorf (Düsseldorf, Germany).
Assessment of positive affect. Positive affect was recorded after each ambulatory blood pressure and heart rate measurement when the participant was asked to give a rating of happiness during the past 5 min on a five-point scale where 1 = low and 5 = high. Ratings of stress, control, and tiredness were also obtained. An average of 33.0 ± 6.8 happiness ratings was obtained from each person on the working day. Monitoring typically began at 0900 hours and ended at 2200 hours, giving a total of 39 possible ratings, so the average response was 84.6%. Nearly half of the ratings (47 ± 37.9%) were positive (scores of 4 or 5 on the five-point scale). We therefore computed the proportion of positive happiness ratings for each individual. This proportion ranged from 0% to 100%. Two hundred sixteen of the 227 men and women who carried out ambulatory monitoring produced adequate happiness data, and they were the individuals analyzed in this study. The remaining 11 people either did not carry out ambulatory monitoring for the complete day and evening or did not fill in the diaries that accompanied cardiovascular measurements. The main analyses of associations between happiness and biological responses were carried out by dividing the sample into quintiles of happiness over the working day (percentage of ratings of 4 and 5) and treating happiness quintile as a between-subject factor in analysis of variance and logistic regression analyses.
Psychological distress measure. Psychological distress was assessed with the GHQ 30, a standard measure designed for use in population studies (23). The Cronbach α in the present sample was 0.93. Scores could range from 0 to 30 with higher scores indicating greater distress. Additionally, a score of 5 or more was defined as a positive GHQ case on the basis of the receiver operating characteristic analysis described by Stansfeld et al. (29), in which it was found that at this threshold, the GHQ has a sensitivity of 72.7% and a specificity of 78% against the clinical interview schedule for the identification of psychological distress of a severity that could be classified as a psychiatric disorder. Laboratory mental stress session. The stress testing session involved assessment of subjective, cardiovascular, and inflammatory responses to the performance of moderately stressful behavioral tasks (26, 30). The two behavioral tasks were computerized color-word interference and mirror tracing. The color-word task involved the presentation of a series of target color words in incongruous colors. At the bottom of the computer screen were four names of colors displayed in incorrect colors, and the task was to press a computer key that corresponded to the position at the bottom of the screen of the name of the color in which the target word was printed. Mirror tracing involved the tracing of a star seen in mirror image with a metal stylus. Participants were told that the average person completed five circuits of the star in the time available and were asked to give accuracy priority over speed on both tasks.
Participants were tested individually in either the morning or afternoon in a light- and temperature-controlled laboratory. They were instructed not to have drunk tea, coffee, or caffeinated beverages or to have smoked for at least two hours before the study, and not to have consumed alcohol or exercised on the evening before or the day of testing. Body weight, height, and waist and hip circumference were measured by a research nurse by using standardized methods. Body mass index (BMI) was computed as weight in kilograms divided by height in meters squared. Data concerning marital status and smoking were collected by questionnaire. Blood pressure and heart rate were monitored continuously from the finger by using a Portapres-2 (TNO-BMI, Amsterdam) (31). A 21-gauge venous cannula was inserted, and the participant rested for 30 min, at the end of which a baseline blood sample was drawn. Blood pressure and heart rate were averaged over minutes 25-30 of the baseline period to constitute resting levels. Participants rated their current level of stress on a seven-point scale from 1 (low) to 7 (high). The two tasks were then administered for 5 min each in random order. After each task, ratings of stress, task involvement, difficulty, and controllability were obtained on seven-point scales. A second blood sample (stress measure) was drawn immediately after the two tasks had been completed. Blood pressure and heart rate monitoring was continuous throughout the task trials. Clottable fibrinogen was measured from frozen samples by an automated Clauss assay in a MDA-180 coagulometer (Organon Teknika, Cambridge, U.K.) by using the manufacturer's reagents and the International Fibrinogen Standard (32). Hematocrit was assessed immediately after each blood sample was drawn by using a microhematocrit centrifuge and reader (Hawksley-Gelman, Lancing, U.K.).
All participants carried out the laboratory stress session first. At the end of the session, the ambulatory monitoring procedure was explained and scheduled for a few days later. The leisure day monitoring of saliva followed the working day assessments.