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An article that was published in the August, 2018 Journal of the American Heart Association studied the effects of sleep duration. They analyzed 74 studies on the topic which included over 3 million participants.
The research indicated that divergence from the recommended 7 to 8 hours of sleep is associated with a higher risk of mortality and cardiovascular events. Longer durations of sleep may be more associated with negative outcomes when compared with shorter sleep durations. They found a 14% increased risk of all-cause mortality for people who get 9 hours of sleep. A 30% increased risk for 10 hours of sleep and a 47% increased risk for 11 hours of sleep. Poor sleep quality was also associated with a 47% increased risk of coronary heart disease. Other factors that go along with long sleep duration include depression, low socioeconomic status, unemployment, and low physical activity.
Short sleep duration was also associated with increases in mortality. Short sleep duration has been shown to increase levels of hormones that increase appetite, caloric intake, and reduced energy expenditure. This facilitates the development of obesity and impaired glycemic control. Cortisol, a stress hormone, also increases with reduced sleep and cardiovascular risk increases due to altered growth hormone metabolism and low-grade inflammation.
In summary, too little AND too much sleep is bad for your health. This study suggests that the optimum amount of time for sleep is 7-8 hours a night.