The person-time for each exposure category (sleep duration of ≤5, 6, 7, 8, or ≥9 hours per night) was accumulated. Incidence rates were calculated by dividing the number of events by person-time of follow-up in each category. Because of small numbers of subjects, we chose to combine the subjects from the 9, 10, and 11 or more hours of sleep groups into one group. In our analysis, each participant contributed person-years to the analysis until an event occurred (either a fatal or a nonfatal MI). Once an event occurred, the participant was removed from the analysis and no longer contributed any further person-years. The relative risk (RR) was computed as the rate in a specific category of exposure divided by that in the reference category (sleep duration of 8 hours per night), with adjustment for age. We chose a reference category of 8 hours per night for 2 reasons. First, 8 hours is conventionally considered to be the appropriate duration of sleep. Second, this category was associated with the lowest rate of CHD in our cohort. In multivariate analyses using pooled logistic regression,9 we simultaneously included age (5-year interval), smoking status (never, past, and current smoking of 1-14, 15-24, and ≥25 cigarettes per day), body mass index (in quintiles), alcohol consumption (0, 1-4, 5-14, and ≥15 g/d), physical activity (weekly energy expenditure in metabolic equivalent hours), menopausal status (premenopausal, postmenopausal without hormone replacement, postmenopausal with past hormone replacement, and postmenopausal with current hormone replacement), depressed mood from 1992 (depression was defined as a 36-Item Short-Form Health Survey mental health index of ≤52), aspirin use (3 categories), parental history of MI before the age of 60 years, and a history of hypercholesterolemia. To control for snoring, we used information from a question contained in the 1986 survey: "Do you snore?" Subjects were asked to check 1 of 3 responses: never, occasionally, or regularly. In our analysis, we used these 3 categories to control for frequency of snoring. We also controlled for the duration of night shifts worked. This information was ascertained from the 1988 questionnaire in which subjects were asked how many years of rotating shifts they performed. Subjects were then divided into 4 groups depending on the duration of night shift working (0, 1-5, 6-14, and ≥15 years). We did not control for a history of hypertension or diabetes mellitus in the primary analyses because decreased sleep duration may be associated with the development of hypertension or diabetes mellitus.5,6 Thus, the development of hypertension or diabetes mellitus may be an intermediate step in the causal pathway between decreased sleep duration and cardiovascular disease. In secondary analyses, we adjusted for both of these variables.