We all know the importance of a good night's sleep, but the time you go to bed could also be crucial, scientists say.
Going to sleep between 10pm and 11pm is associated with a lower risk of developing heart disease compared to other bedtimes, research found.
Staying up into the small hours was particularly bad.
The study found that compared to sleep onset from 10pm to 10.59pm, there was a 25 per cent higher risk of cardiovascular disease with a sleep onset at midnight or later.
There was a 12 per cent greater risk for 11pm to 11.59 pm, and a 24 per cent increased risk for falling asleep before 10pm, according to the study.
Further analysis by sex suggested the association with increased cardiovascular risk was stronger in women, with only sleep onset before 10pm remaining significant for men.
While numerous analyses have investigated the link between sleep duration and cardiovascular disease, the relationship between sleep timing and heart disease is underexplored, researchers said.
Study author Dr David Plans, of the University of Exeter, said: “The body has a 24-hour internal clock, called circadian rhythm, that helps regulate physical and mental functioning.
“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.”
This study examined the association between objectively measured, rather than self-reported, sleep onset in a large sample of adults.
The study included 88,026 people in the UK Biobank study recruited between 2006 and 2010.
The average age was 61 years, with a range between 43 to 79 years, and 58 per cent were women.
Researchers collected data on sleep onset and waking up time over seven days using a device worn on the wrist.
Participants completed demographic, lifestyle, health and physical assessments and questionnaires.
They were then followed up for a new diagnosis of cardiovascular disease, which was defined as a heart attack, heart failure, chronic ischaemic heart disease, stroke and transient ischaemic attack.
During an average follow-up of 5.7 years, 3,172 participants (3.6 per cent) developed cardiovascular disease, according to the study published in European Heart Journal – Digital Health, by the European Society of Cardiology.
According to the paper, the incidence was highest in those with sleep times at midnight or later and lowest in those with sleep onset from 10pm to 10.59 pm.
The researchers analysed the association between sleep onset and cardiovascular events after adjusting for age, sex, sleep duration, sleep irregularity, self-reported chronotype (early bird or night owl), smoking status, body mass index, diabetes, blood pressure, blood cholesterol and socioeconomic status.
Dr Plans said: “Our study indicates that the optimum time to go to sleep is at a specific point in the body’s 24-hour cycle 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.”
He added that the reasons for the observed stronger association between sleep onset and cardiovascular disease in women is unclear.
Dr Plan said: “It may be that there is a sex difference in how the endocrine system responds to a disruption in circadian rhythm.
“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.”
He concluded: “While the findings do not show causality, sleep timing has emerged as a potential cardiac risk factor – independent of other risk factors and sleep characteristics.
“If our findings are confirmed in other studies, sleep timing and basic sleep hygiene could be a low-cost public health target for lowering risk of heart disease.”
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.”