A March 2020 study by the American College of Cardiology suggests that 6-8 hours of sleep is optimal, for heart health. The researchers said,
"The message, based on our findings, is 'sleep well, but not too well.' Getting too little sleep appears bad for your health but too much seems to be harmful as well."
"Unlike other heart disease risk factors such as age or genetics, sleep habits can be adjusted, and even after taking into consideration the impact of established risk factors for atherosclerosis and cardiovascular diseases--for example age, gender, obesity, smoking, hypertension, diabetes, high blood pressure and even a history of coronary artery disease--both short and long sleeping duration may act as additional risk factors."
"We don't fully understand the relationship between sleep and cardiovascular health. It could be that sympathetic nervous system withdrawal or a slowing [of this system] that occurs during sleep may act as a recovery phase for [usual] vascular and cardiac strain."
"Moreover, short sleep duration may be associated with increased cardiovascular risk factors--for example, unhealthy diet, stress, being overweight or greater alcohol consumption--whereas longer sleep duration may be associated with a less active lifestyle pattern and lower physical activity."
"It seems that this amount of sleep may act as an additive cardioprotective factor among people living in modern western societies, and there can be other health benefits to getting sufficient and quality sleep."