Regular 10pm bedtime linked to lower heart risk
There appears to be an optimal bedtime - between 10pm and 11pm - linked to better heart health, say researchers who have studied 88,000 volunteers.
The team behind the UK Biobank work believe synchronising sleep to match our internal body clock may explain the association found with a reduced risk of heart attacks and strokes.
The body's natural 24-hour rhythm is important for wellbeing and alertness.
It can also impact things like blood pressure.
For the study, which was published in the European Heart Journal, the researchers collected data on sleep and wake times over seven days using a wristwatch-like device worn by the volunteers.
And they followed up what happened to the volunteers in terms of heart and circulatory health over an average of six years.
Just over 3,000 of the adults developed cardiovascular disease.
Many of these cases occurred in people who went to bed later or earlier than the "ideal" 10pm to 11pm.
The link persisted after adjustments were made for sleep duration and sleep irregularity.
The researchers tried to look at other factors known to affect a person's heart risk, such as their age, weight and cholesterol levels, but stress their study cannot prove cause and effect.
Study author Dr David Plans, working with the healthtech organisation Huma, said: "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.
"The riskiest time was after midnight, potentially because it may reduce the likelihood of seeing morning light, which resets the body clock."
Regina Giblin, senior cardiac nurse at the British Heart Foundation, said: "This large study suggests that going to sleep between 10 and 11pm could be the sweet spot for most people to keep their heart healthy long-term.
"However, it's important to remember that this study can only show an association and can't prove cause and effect. More research is needed into sleep timing and duration as a risk factor for heart and circulatory diseases.
Getting enough sleep is important for our general wellbeing as well as our heart and circulatory health, and most adults should aim for seven to nine hours of sleep per night, she said.
"But sleep isn't the only factor that can impact heart health. It's also important to look at your lifestyle as knowing your numbers such as blood pressure and cholesterol levels, maintaining a healthy weight and exercising regularly, cutting down on salt and alcohol intake, and eating a balanced diet can also help to keep your heart healthy."