We may have found the right sleep duration for a healthy heart. A study led by scientists from MIT and Harvard published this week by Journal of American College and Cardiology found that the risk of heart attack increased for those who slept for less than 6 hours or more than 9 hours. The findings were independent of any genetical predisposition. 
The study was based on data collected from UK Biobank with 461,347 participants who were free of any relevant cardiovascular diseases. The participants self-reported on the duration of their sleep. Less than 6 hours was deemed as short duration and more than 9 hours as long duration. The team also took into account the quality of sleep and any genetic liabilities.
Short duration sleep (<6 hours) increased the risks for a cardiac attack by 20 percent, while long duration (>9 hours) increased it by 34 percent. The results were found to be independent of other sleep traits. A healthy sleep duration (6-9 hours) could mitigate the risks of a heart attack even for people with a high genetic predisposition.