As the old saying goes, an hour of sleep before midnight is worth two after midnight. 

Now, research finds that getting two hours of sleep before the clock strikes 12 could be key in preventing heart disease. In fact, the large-scale study of more than 88,000 men and women in the U.K., which was just published in the journal European Heart Journal — Digital Health, claims to have hit upon the exact bedtime that’s best for your ticker. And it may impact women the most.

Much has been written about how a third of American adults are sleep-deprived, and that getting less than the recommended seven hours of shut-eye each night is linked with developing heart disease, Type 2 diabetes, dementia and depression. (This costs the U.S. economy $411 billion a year in lost productivity, and Americans were expected to drop $52 billion on sleep aids last year.)

But researchers from the European Society of Cardiology were curious about whether the specific time that someone drifts off to sleep is as important as the length of their sleep. 

Read more: 3 reasons to sleep more: avoid dementia, have great sex and become a better investor

So 88,026 men and women listed in the UK Biobank (a biomedical database) were recruited between 2006 and 2010. The participants, who were ages 43 to 79, wore a sleep tracker on their wrists for a week, and answered questions about their demographics, lifestyle, health and physical assessments. They were then followed up with over a 5.7-year period to see if they were diagnosed with cardiovascular diseases, such as heart attack or stroke. 

Some 3,172 of them (3.6%) did develop something — and it turns out that the highest percentage of heart disease was diagnosed in people who went to bed after midnight. 

But the sweet spot for catching Zs seemed to fall between 10:00 and 10:59 p.m., as this bedtime was linked with lower overall rates of heart disease compared with the earlier and later sleep onset times. In fact, in comparing the bedtimes (which included before 10 p.m., during the 10 o’clock hour, during the 11 o’clock hour, and after midnight) here’s what the researchers found: 

Going to sleep after midnight was associated with a 25% higher risk of heart disease compared with falling sleep between 10:00 and 10:59 p.m. 

Going to sleep between 11:00 and 11:59 p.m. was associated with a 12% higher risk of heart disease compared with falling sleep between 10:00 and 10:59 p.m. 

Going to sleep before 10:00 p.m. was also associated with a 24% higher risk of heart disease compared with falling sleep between 10:00 and 10:59 p.m.

Related: These are the 3 biggest sleep myths

Study author Dr. David Plans of the University of Exeter cautioned that the report doesn’t prove that bedtime causes cardiovascular disease, but rather, that the findings suggest sleep timing could be one important factor in lowering one’s risk for heart disease. The working theory is that disrupting one’s internal body clock, aka your circadian rhythm, by going to bed too early or too late could be what’s responsible for increasing the risk of heart disease. 

“While we cannot conclude causation from our study, the results suggest that early or late bedtimes may be more likely to disrupt the body clock, with adverse consequences for cardiovascular health,” Plans said in a statement. “Our study indicates that the optimum time to go to sleep is at a specific point in the body’s 24-hour cycle [between 10:00 and 10:59 p.m.] and deviations may be detrimental to health. The riskiest time was after midnight, potentially because it may reduce the likelihood of seeing morning light, which resets the body clock.”

Related: No more ‘good’ vs. ‘bad’ foods: 10 healthy eating ‘patterns’ to prevent heart disease and death

Indeed, there’s evidence to suggest that shift workers, for example, are suffering health issues related to their unconventional sleep schedules. This includes an increased risk of certain cancers, a greater risk of heart disease, and a greater mortality risk from all causes. And working at night and sleeping during the day doesn’t just wreak havoc on the circadian rhythm; the night shift lifestyle is also linked with getting insufficient sleep, smoking and eating junk food. And then there’s simple fatigue, which can lead to car accidents, poor decision-making and worsening mood.

Read more: The overnight shift is killing you — so here’s what you can do

What’s more, the association between bedtime and cardiovascular disease was much stronger in women overall in this study. “It may be that there is a sex difference in how the endocrine system responds to a disruption in circadian rhythm,” Plans added. “Alternatively, the older age of study participants could be a confounding factor since women’s cardiovascular risk increases post-menopause –– meaning there may be no difference in the strength of the association between women and men.” 

That’s big news considering cardiovascular disease is the No. 1 killer of women in the U.S., according to the American Heart Association. While men get most of the attention when it comes to cardiovascular disease, it’s actually women who die from heart attacks more frequently. Heart disease is the leading cause of death in both men and women, and being male is a risk factor because men suffer them more frequently. But men are also more likely to survive: 1 in 4 men die of a heart attack each year, compared with 1 in 3 women.

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