The role that regular, healthy sleeping patterns play in staving off disease continues to become clearer through studies that delve into the effects of a disrupted body clock, the 24-hour cycle known as our circadian rhythm. Adding to this pool of knowledge is newly published research into the relationship between bedtimes and cardiovascular disease, which suggests hitting the hay between 10 and 11 pm provides benefits to the heart.
In the past couple of years we've looked at a number of interesting studies digging into the relationship between circadian rhythms and cardiovascular health. One 2019 paper revealed that sleeping less than six hours a night increased a person's risk of early death from heart disease, while a 2020 study suggested that irregular bedtimes day-to-day could double the risk of developing it. Just last month, another study offered clues around why shift workers are more susceptible to heart trouble, detailing new mechanisms within heart cells that guide shifts in cardiac activity across a 24-hour period.
This latest study focuses specifically on what might be the best bedtime for better cardiovascular health. To do so, the authors examined data on 88,000 participants whose sleep and wake times were tracked with a wrist-worn accelerometer over a seven-day period. Assessments and questionnaires were carried out on the participants' demographics, lifestyles and physical health, with the authors then adjusting for factors such as smoking status, body mass index, blood pressure and cholesterol. Adjustments were also made for sleep irregularity, or variations in onset and wake times, and sleep duration, or time the subject spent asleep in a 24-hour period.
The scientists then followed up with the subjects after an average of 5.7 years to identify new diagnoses for cardiovascular disease, of which there were a total of 3,172, or 3.6 percent of the cohort. The incidence was highest in those who went to sleep at midnight or later, who had a 25-percent higher chance of cardiovascular disease than those who went to sleep between 10:00 and 10:59pm. Bedtime between 11:00 and 11:59 pm had 12-percent higher risk, while interestingly, those dozing off before 10 pm had a 24-percent higher risk, though this was only significant among men. These associations persisted after adjusting for sleep irregularity and duration.
“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," says Plans. "The riskiest time was after midnight, potentially because it may reduce the likelihood of seeing morning light, which resets the body clock.”
The scientists also report a stronger relationship between heightened heart disease risk and sleep onset among women, though the reasons behind it are unclear.
“It may be that there is a sex difference in how the endocrine system responds to a disruption in circadian rhythm," says Plans. "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.”
This murkiness around the role of gender speaks to the many variables at play in these studies focusing on association, rather than cause and effect. Were the night owls partaking in lifestyles that could drive up risk or heart disease that weren't reported in their questionnaires, for example? And what other factors might have impacted this risk in the years since the initial seven-day monitoring period? Still, the results have provided the authors with plenty of food for thought, and add weight to the theory that healthy sleep habits can be key to a healthy heart.
“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," says Plans. "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.”
The research was published in the European Heart Journal.
Source: University of Exeter
