Why a single 'healthy sleep' number may not fit both sexes
The relationship between sleep and coronary heart disease is not a straight line. Across 17 studies and more than 900,000 participants, the risk curve is U-shaped for both men and women: risk is elevated at both short and long sleep durations, with a low point in the middle [1]. That overall shape is shared, but the steepness of each arm of the U differs by sex. In women, the short-sleep arm is steeper—shorter sleep is linked to higher CHD risk. In men, the long-sleep arm is steeper—longer sleep is more consequential [1].
This means advice based on a single optimal number can miss the point. The lowest-risk zone is narrow and sex-specific: roughly 7.0–8.0 hours daily for men and 7.5–8.5 hours for women [1]. A woman sleeping 6.5 hours and a man sleeping 6.5 hours are both outside the low-risk zone, but the evidence suggests the woman's CHD risk may be more affected by that shortfall, while a man sleeping 9.5 hours may face a larger penalty than a woman sleeping the same amount [1].
What large population studies see in men and women
A large rural Chinese cohort of nearly 28,000 adults found that short night sleep was associated with CHD, and the association was more obvious in women [2]. Compared with 7 hours of sleep, men sleeping less than 5 hours had an adjusted odds ratio of 1.55 for CHD, while women sleeping less than 5 hours had an odds ratio of 1.80 [2]. (An odds ratio of 1.80 means the odds of CHD were about 80% higher than in the 7-hour reference group.) The same study found that poor sleep quality and short sleep together produced the strongest prevalence of CHD [2].
A six-year prospective study in Iran also reported significant differences between men and women with short and long nightly sleep, and found that very short sleep was associated with a higher incidence of CHD overall [3]. That study did not report a clean split by sex in its abstract, but its finding that men and women differed significantly is consistent with the idea that sex modifies the sleep–CHD link [3].
When the sex difference is small or absent
Not every large study finds a meaningful sex difference. In the China Kadoorie Biobank, which followed about 409,000 adults for nine years, both short (≤5 hours) and long (≥10 hours) sleep were associated with higher risk of major coronary events, and the authors reported 'little differences by sex and age' in those associations [5]. In that cohort, the lowest risk was seen at 7–8 hours of sleep [5].
How can this coexist with the meta-analysis finding clear sex differences? The two results are not necessarily contradictory. The meta-analysis pooled many studies and modeled the full dose-response curve, which is sensitive to where the curve bends for each sex [1]. The Kadoorie study compared broad categories and reported that the overall pattern held similarly in men and women [5]. The difference may therefore be one of degree—how much the short versus long arms of the U diverge by sex—rather than whether any difference exists at all. The meta-analysis, which was designed to test that question directly, found the divergence significant [1].
What this means if you are trying to judge your own risk
The practical takeaway is that sleep duration is a modifiable factor linked to CHD in both sexes, but the risky end of the range is not identical. For women, the evidence points more strongly toward avoiding short sleep; for men, avoiding long sleep may matter more [1]. These are associations from observational studies, not proof that changing sleep duration will change CHD risk, and the meta-analysis pools observational data rather than randomized trials [1].
One unrelated study in the same evidence set examined sex hormone-binding globulin and CHD risk in men and women, but it did not measure sleep, so it cannot inform the sleep question [4]. The sleep-specific evidence is what supports the sex-specific picture: a shared U-shaped curve, with the short-sleep side weighted more heavily for women and the long-sleep side weighted more heavily for men [1][2].
Sources used in this answer
Gender correlation between sleep duration and risk of coronary heart disease: a systematic review and meta-analysis
A meta-analysis of 17 studies and 906,908 participants found a U-shaped relationship between sleep duration and CHD risk in both sexes, with short sleep linked to higher risk in women, long sleep more consequential in men, and optimal duration of 7.0–8.0 hours for men and 7.5–8.5 hours for women.
The association of sleep quality and night sleep duration with coronary heart disease in a large-scale rural population
In a rural Chinese cohort of 27,935 adults, short night sleep was associated with CHD and the association was more obvious in women, with an adjusted odds ratio of 1.80 for women sleeping under 5 hours versus 1.55 for men, compared with 7 hours.
The importance of sleep patterns in the incidence of coronary heart disease: a 6-year prospective study in Mashhad, Iran
A six-year prospective study in Iran found significant differences between men and women with short and long nightly sleep, and very short sleep was associated with increased CHD incidence, especially unstable angina.
Sex Hormone-Binding Globulin and Risk of Coronary Heart Disease in Men and Women
Higher sex hormone-binding globulin levels were associated with lower CHD risk in both men and women, but the study did not examine sleep duration.
Sleep duration and risk of stroke and coronary heart disease: a 9-year community-based prospective study of 0.5 million Chinese adults
In the China Kadoorie Biobank of about 409,000 adults, both short (≤5 hours) and long (≥10 hours) sleep were associated with higher major coronary event risk, with the lowest risk at 7–8 hours and little difference by sex or age.