The association between decreased duration of sleep and incidence of coronary heart disease (CHD) was studied among women enrolled in the Nurses’ Health Study (Ayas et al., 2003). Among women who reported sleeping for 7 or 8 hours per night, there were 541 incident cases of CHD during 451,393 person-years of follow-up. Among those who slept for 6 hours per night there were 267 cases in 175,629 person-years, and among those sleeping 5 or fewer hours per night there were 67 cases during 30,115 person-years of follow-up.
(a) Calculate the incidence rate of CHD among
(i) women who reported sleeping 7–8 hours per night,
(ii) women who reported sleeping for 6 hours per night,
(iii) women who slept 5 or less hours per night, and
(iv) all women.
(b) How strong is the association between sleep duration and the incidence of CHD?
(c) What percentage of CHD cases could theoretically be prevented if all women slept for 7–8 hours per night?
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