It’s long been known that not getting enough sleep is associated with an increased risk of developing several chronic diseases, including heart disease, type 2 diabetes and depression.
A new study, published last Friday in the journal Scientific Reports, suggests, however, that it’s not just how much sleep we get, but the regularity — or irregularity — of our sleep/wake schedule that contributes to that risk of disease.
In fact, the study found that sleep irregularity — not having the same bedtime and wake time every day — was more predictive of an increased risk of heart disease, stroke and metabolic syndrome (high blood pressure, high blood sugar and other conditions that increase the risk of heart disease, stroke and type 2 diabetes) than sleep duration.
“[S]leep irregularity may represent a target for early identification and prevention of cardiometabolic disease,” the study’s authors conclude.
A sleep-deprived nation
Americans are notoriously sleep-deprived. A survey conducted in 2014 by the Centers for Disease Control and Prevention (CDC) found that only 65.2 percent of American adults get seven hours of sleep per night, which is the minimum amount considered necessary for optimal physical and mental health.
Minnesotans seem to do slightly better at getting their shut-eye than those living in most other states. In the CDC survey, 70.8 percent of Minnesotans said they slept for at least seven hours most nights.
The survey didn’t ask people, however, if they had a regular sleep/wake routine.
Research on people who work rotating shifts — and who therefore have rotating bedtimes — has shown strong links between irregular sleep/wake patterns and an increased risk of heart disease, stroke and type 2 diabetes. It’s been unclear, however, if those findings can be generalized to people with unvarying work schedules or to people who are retired.
The authors of the current study, a team of researchers from Duke University Medical Center in Durham, North Carolina, decided to see if they could fill that research gap.
Scoring sleep regularity
For the study, the researchers analyzed data collected from 1,978 older Americans, aged 54 to 93, who were participating in a large, national health study. None of the participants had a diagnosed sleep problem, such as sleep apnea.
For a full week, each participant wore a portable device that tracked when they slept, from the minute they nodded off to the minute they woke up. They also filled out sleep diaries. From this data, the researchers assigned each participant a “sleep regularity index” score.
When the researchers compared those scores to the participants’ health 10 years later, they found some interesting patterns. The participants whose sleep-tracking devices had shown they went to bed and woke up at roughly the same time each day tended to weigh less and to have lower blood pressure and blood sugar than the participants with irregular sleep schedules.
They were also less likely to have had a heart attack or stroke. And they reported fewer symptoms of depression and stress.
The analysis of the data revealed another interesting correlation. People with irregular sleep schedules tended to sleep significantly less during nighttime (an average of 59 minutes less) and more during daytime (65 minutes more) than those with regular schedules.
And when the researchers compared the three key aspects of sleep — duration, timing and regularity — they found that sleep regularity was best at predicting whether someone would develop heart disease and elements of metabolic syndrome.
High blood pressure was, however, more likely among people who slept longer, and obesity was more likely among those who stayed up later.
Limitations and implications
The study comes with several important caveats. First, it is an observational study, and therefore can’t prove a cause-and-effect relationship between regular sleep schedules and the risk of heart disease and metabolic syndrome. In addition, the study’s participants wore a sleep-tracking device for only one week. Their sleep schedules may have changed significantly over the ensuing 10 years.
“From our study, we can’t conclude that sleep irregularity results in health risks, or whether health conditions affect sleep,” said Jessica Lunsford-Avery, the study’s lead author and an assistant professor in psychiatry and behavioral sciences at Duke University, in a released statement. “Perhaps all of these things are impacting each other.”
Still, as Lunsford-Avery and her co-authors point out in their paper, this new study supports previous research that has linked irregular sleep/wake patterns to poorer health.
“The current study suggests relationships between sleep irregularity and cardiometabolic risk extend beyond individuals with rotating shift work schedules to the broader population who may have more stable work patterns and/or have left the work force,” they write.
The researchers say more research is needed on how biology causes changes in sleep regularity — and vice-versa.
“Perhaps there’s something about obesity that disrupts sleep regularity,” Lunsford-Avery said. “Or, as some research suggests, perhaps poor sleep interferes with the body’s metabolism which can lead to weight gain, and it’s a vicious cycle.”
“With more research, we hope to understand what’s going on biologically, and perhaps then we could say what’s coming first or which is the chicken and which is the egg,” she added.
FMI: You can read the study online at the Scientific Reports website.