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It's widely known that we need adequate sleep in order to be healthy. But what is "adequate"? And is more always better? A recent study suggests that a moderate amount of sleep may be optimal for our physical health.

Researchers at the US Centers for Disease Control and Prevention examined a representative sample of over 12,000 adults (ages 30-74). The main outcome variable was excess heart age (EHA), which is defined as the difference between a person's actual age and the "predicted age of the person's vascular system based on their cardiovascular risk profile."

For example, a 55-year-old individual might have a heart and vascular system equal to that of a 60-year-old, which would correspond to an EHA of +5 years. The researchers chose this measure of health because it seems to be a simple way of communicating health risk.

It's important to note that EHA is estimated rather than measured, so we can't know with precision what someone's "real" heart age is. Factors taken into account in estimating EHA include age, , , diabetes, blood pressure, and cholesterol.

Participants were asked how much sleep they got on typical weeknights. Hours of sleep per night were categorized into five groups: 5 or less, 6, 7, 8, and 9 or more. Additional variables were controlled for, including demographics, , socioeconomic status, , body mass index, , and exercise.

As expected, less than 7 hours per night was associated with greater heart age relative to actual age; those who slept 5 hours or less per night had an average heart age 1.4 years older than those sleeping 7 hours per night (EHA = 5.1 vs. 3.7, respectively).

The researchers also found that getting more than 7 hours of sleep per night was linked to greater excess heart age; getting 8 hours of sleep per night was associated with an EHA of 4.5, which was the same as among those getting 6 hours per night. The effect of sleep duration on heart age was stronger in women than in men.

What should we make of these findings? The study authors note that it's important to attend to sleep health even if we exercise and have a good diet, as the results controlled for these and other health variables. They also suggest that short sleep may promote "chronic, low-level inflammation," thereby affecting cardiovascular health; short sleep may also lead to increased and subsequent .

The researchers hypothesized that variables like unemployment and impaired mental health, among others, could explain the health risk for longer sleepers. They also note that previous studies have found mixed results for the health associations of getting 8 hours sleep per night.

For those who sleep 8 hours or more per night, should they start aiming for 7 hours based on these new results? Probably not. For one, this study was observational rather than experimental, which means participants weren't randomly assigned to hours of sleep per night. Thus we can't determine causation from this association, as it's just as likely that health status causes sleep duration as the reverse.

There also may be other variables that go along with sleep duration that weren't measured or completely controlled for that drove the observed effects. For example, the researchers note that people who slept 8 hours per night versus 7 were more likely to smoke and have diabetes.

What this study does underscore is that we shouldn't assume sleeping 8 hours or more per night is an indicator of good health, or that people who sleep 7 hours per night would be better off sleeping 8 or more. As with diet and physical exercise, moderation may be key.