“The body has a 24-hour internal clock, called circadian rhythm,” explains Dr. Plans, “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.”
“Certainly, the importance of circadian rhythms and body clock in human health is pretty clear. We all have a 24-hour clock that affects various body areas,” Dr. Atul Malhotra, a University of California San Diego sleep medicine specialist, told Medical News Today.
“But whether people go to sleep early or go to sleep late or something is kind of an individual characteristic,” Dr. Malhotra added.
“Some of us are early birds, and some of us are night owls, and that’s just the way it is. So to make a blanket statement that everybody should go to bed at X time is a bit silly, because, you know, it depends on your individual biology.”
Dr. Malhotra told MNT that, for example, while research has shown that heart attacks tend to happen at around 10:00 a.m., it is not about the time shown by the clock on the wall. “After you wake up in the morning,” he explained, “is when the spike tends to occur based on [one’s] intrinsic clock. It’s not just the external clock that matters.”
Tracking bedtime
In total, there were 88,026 participants in the study — 58% of them female — recruited by the UK Biobank between 2006 and 2010. They ranged in age from 43 to 70 years. The mean age was 61 years.
The participants filled out questionnaires reporting their age, sex, sociodemographic status, the amount of sleep they were used to, their experience of interrupted sleep, their chronotype — night owl or early bird — and whether they smoked.
Moreover, the researchers assessed the individuals’ body mass index, blood pressure and cholesterol levels, and presence of diabetes.
They then outfitted each study participant with a wrist accelerometer that allowed for precise tracking of the onset of sleep according to the individuals’ lack of movement.
Over 7 days and nights, the researchers collected initial sleeping data.
An extended follow-up period averaging 5.7 years tracked the individuals’ development of CVD.
The researchers considered heart attack, heart failure, stroke, chronic ischemic heart disease, and transient ischemic attack to be examples of CVD.
The results
During the follow-up period, 3,172, or 3.6%, of the participants developed CVD. The study authors found that CVD risk was different for females than for males.
The lowest incidence of CVD was present in females who went to bed between 10:00 and 10:59 p.m.
Compared with that group, the females who turned in between 11:00 and 11:59 p.m. were 12% more likely to have developed CVD.
The researchers associated the highest risk with going to sleep after midnight (25%) or before 10:00 p.m. (24%).
For males, the only strong association with a higher risk of CVD was among those who went to sleep before 10:00 p.m.
Speculating on the difference between the sexes, Dr. Plans says:
“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 postmenopause — meaning there may be no difference in the strength of the association between women and men.”
Dr. Malhotra suggested that the study findings may be influenced by a range of confounding factors. When it comes to the apparent link between early sleep onset and CVD, he said: “My guess is that some of the people who fall asleep earlier are sleep-deprived. They’re not getting enough sleep at night, so they tend to fall asleep early.”
“Sleep apnea is another common condition, and that could be another confounder,” said Dr. Malhotra, pointing out that it “can affect people’s cardiovascular risk.”
- “While the findings do not show causality,” Dr. Plans concludes, “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.”
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